CSS: The Definitive Guide (2017)

Chapter 15. Selectors

One of the primary advantages of CSS—particularly to designers—is its ability to easily apply a set of styles to all elements of the same type. Unimpressed? Consider this: by editing a single line of CSS, you can change the colors of all your headings. Don’t like the blue you’re using? Change that one line of code, and they can all be purple, yellow, maroon, or any other color you desire. That lets you, the designer, focus on design, rather than grunt work. The next time you’re in a meeting and someone wants to see headings with a different shade of green, just edit your style and hit Reload. Voilà! The results are accomplished in seconds and there for everyone to see.

Of course, CSS can’t solve all your problems—you can’t use it to change the colorspace of your PNGs, for example, at least not yet—but it can make some global changes much easier. So let’s begin with selectors and structure.

Basic Style Rules

As stated, a central feature of CSS is its ability to apply certain rules to an entire set of element types in a document. For example, let’s say that you want to make the text of all h2 elements appear gray. Using old-school HTML, you’d have to do this by inserting <font color="gray">…</font> tags in all your h2 elements. Using the style attribute, which is also bad practice, would require you to include style="color: grey;" in all your h2 elements:

<h2><font color="gray">This is h2 text</font></h2>

<h2 style="color: gray;">This is h2 text</h2>

Obviously, this is a tedious process if your document contains a lot of h2 elements. Worse, if you later decide that you want all those h2s to be green instead of gray, you’d have to start the manual tagging all over again. (Yes, this is really how it used to be done!)

CSS allows you to create rules that are simple to change, edit, and apply to all the text elements you define (the next section will explain how these rules work). For example, simply write this rule once to make all your h2 elements gray:

h2 {color: gray;}

If you want to change all h2 text to another color—say, silver—simply alter the value:

h2 {color: silver;}

Element Selectors

An element selector is most often an HTML element, but not always. For example, if a CSS file contains styles for an XML document, element selectors might look something like this:

quote {color: gray;}

bib {color: red;}

booktitle {color: purple;}

myElement {color: red;}

In other words, the elements of the document serve as the most basic selectors. In XML, a selector could be anything, since XML allows for the creation of new markup languages that can have just about anything as an element name. If you’re styling an HTML document, on the other hand, the selector will generally be one of the many HTML elements such as p, h3, em, a, or even html itself. For example:

html {color: black;}

h1 {color: gray;}

h2 {color: silver;}

The results of this style sheet are shown in Figure 15-1.

Figure 15-1. Simple styling of a simple document

Once you’ve globally applied styles directly to elements, you can shift those styles from one element to another. Let’s say you decide that the paragraph text, not the h1 elements, in Figure 15-1 should be gray. No problem. Simply change the h1 selector to p:

html {color: black;}

p {color: gray;}

h2 {color: silver;}

The results are shown in Figure 15-2.

Figure 15-2. Moving a style from one element to another

Declarations and Keywords

The declaration block contains one or more declarations. A declaration is always formatted as a property followed by a colon and then a value followed by a semicolon. The colon and semicolon can be followed by zero or more spaces. In nearly all cases, a value is either a single keyword or a space-separated list of one or more keywords that are permitted for that property. If you use an incorrect property or value in a declaration, the whole rule will be ignored. Thus, the following two declarations would fail:

brain-size: 2cm; /* unknown property 'brain-size' */

color: ultraviolet; /* unknown value 'ultraviolet' */

In an instance where you can use more than one keyword for a property’s value, the keywords are usually separated by spaces, with some cases requiring slashes (/) or commas. Not every property can accept multiple keywords, but many, such as the font property, can. Let’s say you want to define medium-sized Helvetica for paragraph text, as illustrated in Figure 15-3.

Figure 15-3. The results of a property value with multiple keywords

The rule would read as follows:

p {font: medium Helvetica;}

Note the space between medium and Helvetica, each of which is a keyword (the first is the font’s size and the second is the actual font name). The space allows the user agent to distinguish between the two keywords and apply them correctly. The semicolon indicates that the declaration has been concluded.

These space-separated words are referred to as keywords because, taken together, they form the value of the property in question. For instance, consider the following fictional rule:

rainbow: red orange yellow green blue indigo violet;

There is no such property as rainbow, of course, but the example is useful for illustrative purposes. The value of rainbow is red orange yellow green blue indigo violet, and the seven keywords add up to a single, unique value. We can redefine the value for rainbow as follows:

rainbow: infrared red orange yellow green blue indigo violet ultraviolet;

Now we have a new value for rainbow composed of nine keywords instead of seven. Although the two values look mostly the same, they are as unique and different as zero and one. This may seem an abstract point, but it’s critical to understanding some of the subtler effects of specificity and the cascade (covered in later in this book).

As we’ve seen, CSS keywords are usually separated by spaces—with a few exceptions in one instance.

In CSS2.1 there was one exception: in the CSS property font, there is exactly one place where a forward slash (/) could be used to separate two specific keywords. Here’s an example:

h2 {font: large/150% sans-serif;}

The slash separates the keywords that set the element’s font size and line height. This is the only place the slash is allowed to appear in the font declaration. All of the other keywords allowed for font are separated by spaces.

The slash also makes an appearence in the CSS border-radius shorthand property between the vertical and horizontal declarations.

div {border-radius: 1em / 2em; }

There are also some keywords that are separated by commas. When declaring multiple values, such as multiple background images, transition properties and shadows, the declarations are separated with commas. Additionally, parameters in functions, such as linear gradients and transforms, are comma separated.

.box {box-shadow: inset -1px -1px white,

3px 3px 3px rgba(0,0,0,0.2);

background-image: url(myimage.png),

linear-gradient(180deg, #FFF 0%, #000 100%);

transform: translate(100px, 200px);

}

a:hover {transition: color, background-color 200ms ease-in 50ms;}

Those are the basics of simple declarations, but they can get much more complex. The next section begins to show you just how powerful CSS can be.

Grouping

So far, we’ve seen fairly simple techniques for applying a single style to a single selector. But what if you want the same style to apply to multiple elements? If that’s the case, you’ll want to use more than one selector or apply more than one style to an element or group of elements.

Grouping Selectors

Let’s say you want both h2 elements and paragraphs to have gray text. The easiest way to accomplish this is to use the following declaration:

h2, p {color: gray;}

By placing the h2 and p selectors on the left side of the rule and separating them with a comma, you’ve defined a rule where the style on the right (color: gray;) applies to the elements referenced by both selectors. The comma tells the browser that there are two different selectors involved in the rule. Leaving out the comma would give the rule a completely different meaning, which we’ll explore later in “Descendant Selectors”.

There are really no limits to how many selectors you can group together. For example, if you want to display a large number of elements in gray, you might use something like the following rule:

body, table, th, td, h1, h2, h3, h4, p, pre, strong, em, b, i {color: gray;}

Grouping allows an author to drastically compact certain types of style assignments, which makes for a shorter style sheet. The following alternatives produce exactly the same result, but it’s pretty obvious which one is easier to type:

h1 {color: purple;}

h2 {color: purple;}

h3 {color: purple;}

h4 {color: purple;}

h5 {color: purple;}

h6 {color: purple;}

h1, h2, h3, h4, h5, h6 {color: purple;}

Grouping allows for some interesting choices. For example, all of the groups of rules in the following example are equivalent—each merely shows a different way of grouping both selectors and declarations:

/* group 1 */

h1 {color: silver; background: white;}

h2 {color: silver; background: gray;}

h3 {color: white; background: gray;}

h4 {color: silver; background: white;}

b {color: gray; background: white;}

/* group 2 */

h1, h2, h4 {color: silver;}

h2, h3 {background: gray;}

h1, h4, b {background: white;}

h3 {color: white;}

b {color: gray;}

/* group 3 */

h1, h4 {color: silver; background: white;}

h2 {color: silver;}

h3 {color: white;}

h2, h3 {background: gray;}

b {color: gray; background: white;}

Any of these will yield the result shown in Figure 15-4. (These styles use grouped declarations, which are explained in an upcoming section, “Grouping Declarations”.)

Figure 15-4. The result of equivalent style sheets

The universal selector

CSS2 introduced a new simple selector called the universal selector, displayed as an asterisk (*). This selector matches any element at all, much like a wildcard. For example, to make every single element in a document red, you would write:

* {color: red;}

This declaration is equivalent to a grouped selector that lists every single element contained within the document. The universal selector lets you assign the color value red to every element in the document in one efficient stroke. Beware, however: although the universal selector is convenient, with a specificity on 0-0-0, and by nature of the fact that it targets everything within it’s declaration scope, it can have unintended consequences, which are discussed later in this book.

Grouping Declarations

Since you can group selectors together into a single rule, it follows that you can also group declarations. Assuming that you want all h1 elements to appear in purple, 18-pixel-high Helvetica text on an aqua background (and you don’t mind blinding your readers), you could write your styles like this:

h1 {font: 18px Helvetica;}

h1 {color: purple;}

h1 {background: aqua;}

But this method is inefficient—imagine creating such a list for an element that will carry 10 or 15 styles! Instead, you can group your declarations together:

h1 {font: 18px Helvetica; color: purple; background: aqua;}

This will have exactly the same effect as the three-line style sheet just shown.

Note that using semicolons at the end of each declaration is crucial when you’re grouping them. Browsers ignore whitespace in style sheets, so the user agent must rely on correct syntax to parse the style sheet. You can fearlessly format styles like the following:

h1 {

font: 18px Helvetica;

color: purple;

background: aqua;

}

You can also minimize your CSS, removing all non-required spaces.

h1{font:18px Helvetica;color:purple;background:aqua;}

Ignoring whitespace, the above three are treated equally by the server, but the second one is most human readable, and the recommended method of marking up your CSS during development.

If the semicolon is omitted on the second statement, the user agent will interpret the style sheet as follows:

h1 {

font: 18px Helvetica;

color: purple background: aqua;

}

Because background: is not a valid value for color, and because color can be given only one keyword, a user agent will ignore the color declaration (including the background: aqua part) entirely. You might think the browser would at least render h1s as purple text without an aqua background, but not so. Instead, they will be the default color (which is usually black) with a transparent background (which is also a default). The declaration font: 18px Helveticawill still take effect since it was correctly terminated with a semicolon.

Note

Although it is not technically necessary to follow the last declaration of a rule with a semicolon in CSS, it is generally good practice to do so. First, it will keep you in the habit of terminating your declarations with semicolons, the lack of which is one of the most common causes of rendering errors. Second, if you decide to add another declaration to a rule, you won’t have to worry about forgetting to insert an extra semicolon. Third, if you ever use a preprocessor like Sass, trailing semicolons are often required for all declarations. Avoid all these problems—always follow a declaration with a semicolon, wherever the rule appears.

As with selector grouping, declaration grouping is a convenient way to keep your style sheets short, expressive, and easy to maintain.

Grouping Everything

You now know that you can group selectors and you can group declarations. By combining both kinds of grouping in single rules, you can define very complex styles using only a few statements. Now, what if you want to assign some complex styles to all the headings in a document, and you want the same styles to be applied to all of them? Here’s how to do it:

h1, h2, h3, h4, h5, h6 {color: gray; background: white; padding: 0.5em;

border: 1px solid black; font-family: Charcoal, sans-serif;}

You’ve grouped the selectors, so the styles on the right side of the rule will be applied to all the headings listed; grouping the declarations means that all of the listed styles will be applied to the selectors on the left side of the rule. The result of this rule is shown in Figure 15-5.

Figure 15-5. Grouping both selectors and rules

This approach is preferable to the drawn-out alternative, which would begin with something like this:

h1 {color: gray;}

h2 {color: gray;}

h3 {color: gray;}

h4 {color: gray;}

h5 {color: gray;}

h6 {color: gray;}

h1 {background: white;}

h2 {background: white;}

h3 {background: white;}

…and continue for many lines. You can write out your styles the long way, but I wouldn’t recommend it—editing them would be as tedious as using style attributes everywhere!

It’s possible to add even more expression to selectors and to apply styles in a way that cuts across elements in favor of types of information. Of course, to get something so powerful, you’ll have to do a little work in return, but it’s well worth it.

With updates to HTML, like the HTML5 specification, new elements have come about. Some browsers predate and don’t support some of these newer elements. Versions of Internet Explorer prior to IE9, did not support selecting elements they did not understand when using element selectors based on elements they did not recognize. The solution was to create the element in the DOM, thereby informing the browser that said element my exist. For example, IE8 does not recognize the <main> element.

document.createElement('main');

By creating the element with createElement, older versions of Internet Explorer will recognize the existence of the element, allowing it to be selected and styled.

Class and ID Selectors

So far, we’ve been grouping selectors and declarations together in a variety of ways, but the selectors we’ve been using are very simple ones that refer only to document elements. Element selectors are fine up to a point, but there are times when you need something a little more specialized.

In addition to raw document elements, there are class selectors and ID selectors, which let you assign styles in a way that is independent of document elements. These selectors can be used on their own or in conjunction with element selectors. However, they work only if you’ve marked up your document appropriately, so using them generally involves a little forethought and planning.

For example, say you’re drafting a document that discusses ways of handling plutonium. The document contains various warnings about safely dealing with such a dangerous substance. You want each warning to appear in boldface text so that it will stand out. However, you don’t know which elements these warnings will be. Some warnings could be entire paragraphs, while others could be a single item within a lengthy list or a small section of text. So, you can’t define a rule using element selectors of any kind. Suppose you tried this route:

p {

font-weight: bold;

color: red;

}

All paragraphs would be red and bold, not just those that contain warnings. You need a way to select only the text that contains warnings, or more precisely, a way to select only those elements that are warnings. How do you do it? You apply styles to parts of the document that have been marked in a certain way, independent of the elements involved, by using class selectors.

Class Selectors

The most common way to apply styles without worrying about the elements involved is to use class selectors. Before you can use them, however, you need to modify your actual document markup so that the class selectors will work. Enter the class attribute:

<p class="warning">When handling plutonium, care must be taken to avoid

the formation of a critical mass.</p>

<p>With plutonium, <span class="warning">the possibility of implosion is

very real, and must be avoided at all costs</span>. This can be accomplished

by keeping the various masses separate.</p>

To associate the styles of a class selector with an element, you must assign a class attribute to the appropriate value. In the previous code, a class value of warning was assigned to two elements: the first paragraph and the spanelement in the second paragraph.

All you need now is a way to apply styles to these classed elements. In HTML documents, you can use a very compact notation where the name of a class is preceded by a period (.) and can be joined with an element selector:

.warning {font-weight: bold;}

When combined with the example markup shown earlier, this simple rule has the effect shown in Figure 15-6. That is, the declaration font-weight: bold will be applied to every element (thanks to the presence of the implicit universal selector) that carries a class attribute with a value of warning.

Note

=== The universal selector, represented by *, is implied when an ID, class, attribute selector, pseudo-class or pseudo-element selector is written without being attached to an element selector. ===

Figure 15-6. Using a class selector

As you can see, the class selector works by directly referencing a value that will be found in the class attribute of an element. This reference is always preceded by a period (.), which marks it as a class selector. The period helps keep the class selector separate from anything with which it might be combined—such as an element selector. For example, you may want boldface text only when an entire paragraph is a warning:

p.warning {font-weight: bold;}

The selector now matches any p elements that have a class attribute containing the word warning, but no other elements of any kind, classed or otherwise. Since the span element is not a paragraph, the rule’s selector doesn’t match it, and it won’t be displayed using boldfaced text.

If you did want to assign different styles to the span element, you could use the selector span.warning:

p.warning {font-weight: bold;}

span.warning {font-style: italic;}

In this case, the warning paragraph is boldfaced, while the warning span is italicized. Each rule applies only to a specific type of element/class combination, so it does not leak over to other elements.

Another option is to use a combination of a general class selector and an element-specific class selector to make the styles even more useful, as in the following markup:

.warning {font-style: italic;}

span.warning {font-weight: bold;}

The results are shown in Figure 15-7.

Figure 15-7. Using generic and specific selectors to combine styles

In this situation, any warning text will be italicized, but only the text within a span element with a class of warning will be both boldfaced and italicized.

Notice the format of the general class selector in the previous example: it’s simply a class name preceded by a period without any element name, and no universal selector. In cases where you only want to select all elements that share a class name, you can omit the universal selector from a class selector without any ill effects.

Multiple Classes

In the previous section, we dealt with class values that contained a single word. In HTML, it’s possible to have a space-separated list of words in a single class value. For example, if you want to mark a particular element as being both urgent and a warning, you could write:

<p class="urgent warning">When handling plutonium, care must be taken to

avoid the formation of a critical mass.</p>

<p>With plutonium, <span class="warning">the possibility of implosion is

very real, and must be avoided at all costs</span>. This can be accomplished

by keeping the various masses separate.</p>

The order of the words doesn’t actually matter; warning urgent would also suffice and would yield precisely the same results no matter how the CSS class attribute is written.

Now let’s say you want all elements with a class of warning to be boldfaced, those with a class of urgent to be italic, and those elements with both values to have a silver background. This would be written as follows:

.warning {font-weight: bold;}

.urgent {font-style: italic;}

.warning.urgent {background: silver;}

By chaining two class selectors together, you can select only those elements that have both class names, in any order. As you can see, the HTML source contains class="urgent warning" but the CSS selector is written .warning.urgent. Regardless, the rule will still cause the “When handling plutonium . . . ” paragraph to have a silver background, as illustrated in Figure 15-8. This happens because the order the words are written in doesn’t matter. (This is not to say the order of classes is always irrelevant, but we’ll get to that later in the book.)

Figure 15-8. Selecting elements with multiple class names

If a multiple class selector contains a name that is not in the space-separated list, then the match will fail. Consider the following rule:

p.warning.help {background: red;}

As you would expect, the selector will match only those p elements with a class containing the words warning and help. Therefore, it will not match a p element with just the words warning and urgent in its class attribute. It would, however, match the following:

<p class="urgent warning help">Help me!</p>

ID Selectors

In some ways, ID selectors are similar to class selectors, but there are a few crucial differences. First, ID selectors are preceded by an octothorpe (#)—also known as a pound sign (in the US), hash sigh,hash mark, or tic-tac-toe board—instead of a period. Thus, you might see a rule like this one:

*#first-para {font-weight: bold;}

This rule produces boldfaced text in any element whose id attribute has a value of first-para.

The second difference is that instead of referencing values of the class attribute, ID selectors refer, unsurprisingly, to values found in id attributes. Here’s an example of an ID selector in action:

*#lead-para {font-weight: bold;}

<p id="lead-para">This paragraph will be boldfaced.</p>

<p>This paragraph will NOT be bold.</p>

Note that the value lead-para could have been assigned to any element within the document. In this particular case, it is applied to the first paragraph, but you could have applied it just as easily to the second or third paragraph.

As with class selectors, it is possible to omit the universal selector from an ID selector. In the previous example, you could also have written:

#lead-para {font-weight: bold;}

The effect of this selector would be the same.

Another similarity between classes and IDs is that IDs can also be selected independently of an element. There may be circumstances in which you know that a certain ID value will appear in a document, but you don’t know the element on which it will appear (as in the plutonium-handling warnings), so you’ll want to declare standalone ID selectors. For example, you may know that in any given document, there will be an element with an ID value of mostImportant. You don’t know whether that most important thing will be a paragraph, a short phrase, a list item, or a section heading. You know only that it will exist in each document, occur in an arbitrary element, and appear no more than once. In that case, you would write a rule like this:

#mostImportant {color: red; background: yellow;}

This rule would match any of the following elements (which, as noted before, should not appear together in the same document because they all have the same ID value):

<h1 id="mostImportant">This is important!</h1>

<em id="mostImportant">This is important!</em>

<ul id="mostImportant">This is important!</ul>

While ids are limited to a single occurrence per document, if the above were erroneously included in a single document, all three would have red text with a yellow background. Repeating the same id value within a page will create invalid code, and may cause broken JavaScript functionality, browsers will still style all matched elements.

Deciding Between Class and ID

You may assign classes to any number of elements, as demonstrated earlier; the class name warning was applied to both a p and a span element, and it could have been applied to many more elements. IDs, on the other hand, are used once, and only once, within an HTML document. Therefore, if you have an element with an id value of lead-para, no other element in that document can have an id value of lead-para.

Note

In the real world, browsers don’t always check for the uniqueness of IDs in HTML. That means that if you sprinkle an HTML document with several elements, all of which have the same value for their ID attributes, you’ll probably get the same styles applied to each. This is incorrect behavior, but it happens anyway. Having more than one of the same ID value in a document also makes DOM scripting more difficult, since functions like getElementById()depend on there being one, and only one, element with a given ID value.

Unlike class selectors, ID selectors can’t be combined with other IDs, since ID attributes do not permit a space-separated list of words.

Another difference between class and id names is that IDs carry more weight when you’re trying to determine which styles should be applied to a given element. This will be explained in greater detail later on.

Also note that class and ID selectors may be case-sensitive, depending on the document language. HTML defines class and ID values to be case-sensitive, so the capitalization of your class and ID values must match that found in your documents. Thus, in the following pairing of CSS and HTML, the elements text will not be boldfaced:

p.criticalInfo {font-weight: bold;}

<p class="criticalinfo">Don't look down.</p>

Because of the change in case for the letter i, the selector will not match the element shown.

On a purely syntactical level, the dot-class notation (e.g., .warning) is not guaranteed to work for XML documents. As of this writing, the dot-class notation works in HTML, SVG, and MathML, and it may well be permitted in future languages, but it’s up to each language’s specification to decide that. The hash-ID notation (e.g., #lead) will work in any document language that has an attribute that enforces uniqueness within a document. Uniqueness can be enforced with an attribute called id, or indeed anything else, as long as the attribute’s contents are defined to be unique within the document.

Attribute Selectors

When it comes to both class and ID selectors, what you’re really doing is selecting values of attributes. The syntax used in the previous two sections is particular to HTML, XHTML, SVG, and MathML documents (as of this writing). In other markup languages, these class and ID selectors may not be available (as, indeed, those attributes may not be present). To address this situation, CSS2 introduced attribute selectors, which can be used to select elements based on their attributes and the values of those attributes. There are four general types of attribute selectors: simple attribute selectors, exact attribute value selectors, partial-match attribute value selectors, and leading-value attribute selectors.

Simple Attribute Selectors

If you want to select elements that have a certain attribute, regardless of that attribute’s value, you can use a simple attribute selector. For example, to select all h1 elements that have a class attribute with any value and make their text silver, write:

h1[class] {color: silver;}

So, given the following markup:

<h1 class="hoopla">Hello</h1>

<h1>Serenity</h1>

<h1 class="fancy">Fooling</h1>

…you get the result shown in Figure 15-9.

Figure 15-9. Selecting elements based on their attributes

This strategy is very useful in XML documents, as XML languages tend to have element and attribute names that are very specific to their purpose. Consider an XML language that is used to describe planets of the solar system (we’ll call it PlanetML). If you want to select all pml-planet elements with a moons attribute and make them boldface, thus calling attention to any planet that has moons, you would write:

pml-planet[moons] {font-weight: bold;}

This would cause the text of the second and third elements in the following markup fragment to be boldfaced, but not the first:

<pml-planet>Venus</pml-planet>

<pml-planet moons="1">Earth</pml-planet>

<pml-planet moons="2">Mars</pml-planet>

In HTML documents, you can use this feature in a number of creative ways. For example, you could style all images that have an alt attribute, thus highlighting those images that are correctly formed:

img[alt] {border: 3px solid red;}

(This particular example is generally useful more for diagnostic purposes—that is, determining whether images are indeed correctly marked up—than for design purposes.)

If you wanted to boldface any element that includes title information, which most browsers display as a “tool tip” when a cursor hovers over the element, you could write:

*[title] {font-weight: bold;}

Similarly, you could style only those anchors (a elements) that have an href attribute, thus applying the styles to any hyperlink but not to any placeholder anchors.

It is also possible to select based on the presence of more than one attribute. You do this simply by chaining the attribute selectors together. For example, to boldface the text of any HTML hyperlink that has both an href and a titleattribute, you would write:

a[href][title] {font-weight: bold;}

This would boldface the first link in the following markup, but not the second or third:

<a href="http://www.w3.org/" title="W3C Home">W3C</a><br />

<a href="http://www.webstandards.org">Standards Info</a><br />

<a title="Not a link">dead.letter</a>

Selection Based on Exact Attribute Value

You can further narrow the selection process to encompass only those elements whose attributes are a certain value. For example, let’s say you want to boldface any hyperlink that points to a certain document on the web server. This would look something like:

a[href="http://www.css-discuss.org/about.html"] {font-weight: bold;}

This will boldface the text of any a element that has an href attribute with exactly the value http://www.css-discuss.org/about.html. Any change at all, even dropping the www. part or changing to a secure protocol with https, will prevent a match.

Any attribute and value combination can be specified for any element. However, if that exact combination does not appear in the document, then the selector won’t match anything. Again, XML languages can benefit from this approach to styling. Let’s return to our PlanetML example. Suppose you want to select only those planet elements that have a value of 1 for the attribute moons:

planet[moons="1"] {font-weight: bold;}

This would boldface the text of the second element in the following markup fragment, but not the first or third:

<planet>Venus</planet>

<planet moons="1">Earth</planet>

<planet moons="2">Mars</planet>

As with attribute selection, you can chain together multiple attribute-value selectors to select a single document. For example, to double the size of the text of any HTML hyperlink that has both an href with a value of http://www.w3.org/ and a title attribute with a value of W3C Home, you would write:

a[href="http://www.w3.org/"][title="W3C Home"] {font-size: 200%;}

This would double the text size of the first link in the following markup, but not the second or third:

<a href="http://www.w3.org/" title="W3C Home">W3C</a><br />

<a href="http://www.webstandards.org"

title="Web Standards Organization">Standards Info</a><br />

<a href="http://www.example.org/" title="W3C Home">dead.link</a>

The results are shown in Figure 15-10.

Figure 15-10. Selecting elements based on attributes and their values

Again, this format requires an exact match for the attribute’s value. Matching becomes an issue when the selector form encounters values that can in turn contain a space-separated list of values (e.g., the HTML attribute class). For example, consider the following markup fragment:

<planet type="barren rocky">Mercury</planet>

The only way to match this element based on its exact attribute value is to write:

planet[type="barren rocky"] {font-weight: bold;}

If you were to write planet[type="barren"], the rule would not match the example markup and thus would fail. This is true even for the class attribute in HTML. Consider the following:

<p class="urgent warning">When handling plutonium, care must be taken to

avoid the formation of a critical mass.</p>

To select this element based on its exact attribute value, you would have to write:

p[class="urgent warning"] {font-weight: bold;}

This is not equivalent to the dot-class notation covered earlier, as we will see in the next section. Instead, it selects any p element whose class attribute has exactly the value "urgent warning", with the words in that order and a single space between them. It’s effectively an exact string match.

Also, be aware that ID selectors and attribute selectors that target the id attribute are not precisely the same. In other words, there is a subtle but crucial difference between h1#page-title and h1[id="page-title"]. This difference is explained in the next chapter in the section on specificity.

Selection Based on Partial Attribute Values

Odds are that you’ll want to select elements based on portions of their attribute values, rather than the full value. For such situations, CSS actually offers a variety of options for matching substrings in an attribute’s value. These are summarized in Table 15-1.

A Particular Attribute Selection Type

The first of these attribute selectors that match on a partial subset of an element’s attribute value is actually easier to show than it is to describe. Consider the following rule:

*[lang|="en"] {color: white;}

This rule will select any element whose lang attribute is equal to en or begins with en-. Therefore, the first three elements in the following example markup would be selected, but the last two would not:

<h1 lang="en">Hello!</h1>

<p lang="en-us">Greetings!</p>

<div lang="en-au">G'day!</div>

<p lang="fr">Bonjour!</p>

<h4 lang="cy-en">Jrooana!</h4>

In general, the form [att|="val"] can be used for any attribute and its values. Let’s say you have a series of figures in an HTML document, each of which has a filename like figure-1.gif and figure-3.jpg. You can match all of these images using the following selector:

img[src|="figure"] {border: 1px solid gray;}

Or, if you’re creating a CSS framework or pattern library, instead of creating redundant classes like "btn btn-small btn-arrow btn-active" you can declare "btn-small-arrow-active", and targeting the class of elements with

*[class|="btn"] { border-radius: 5px;}

<button class="btn-small-arrow-active">Click Me</button>

The most common use for this type of attribute selector is to match language values, as demonstrated later in this chapter.

Matching one word in a space-separated list

For any attribute that accepts a space-separated list of words, it is possible to select elements based on the presence of any one of those words. The classic example in HTML is the class attribute, which can accept one or more words as its value. Consider our usual example text:

<p class="urgent warning">When handling plutonium, care must be taken to

avoid the formation of a critical mass.</p>

Let’s say you want to select elements whose class attribute contains the word warning. You can do this with an attribute selector:

p[class~="warning"] {font-weight: bold;}

Note the presence of the tilde (~) in the selector. It is the key to selection based on the presence of a space-separated word within the attribute’s value. If you omit the tilde, you would have an exact value matching attribute selector, as discussed in the previous section.

This selector construct is equivalent to the dot-class notation discussed earlier in the chapter. Thus, p.warning and p[class~="warning"] are equivalent when applied to HTML documents. Here’s an example that is an HTML version of the “PlanetML” markup seen earlier:

<span class="barren rocky">Mercury</span>

<span class="cloudy barren">Venus</span>

<span class="life-bearing cloudy">Earth</span>

To italicize all elements with the word barren in their class attribute, you write:

span[class~="barren"] {font-style: italic;}

This rule’s selector will match the first two elements in the example markup and thus italicize their text, as shown in Figure 15-11. This is the same result we would expect from writing span.barren {font-style: italic;}.

Figure 15-11. Selecting elements based on portions of attribute values

So why bother with the tilde-equals attribute selector in HTML? Because it can be used for any attribute, not just class. For example, you might have a document that contains a number of images, only some of which are figures. You can use a partial-match value attribute selector aimed at the title text to select only those figures:

img[title~="Figure"] {border: 1px solid gray;}

This rule will select any image whose title text contains the word Figure. Therefore, as long as all your figures have title text that looks something like “Figure 4. A bald-headed elder statesman,” this rule will match those images. For that matter, the selector img[title~="Figure"] will also match a title attribute with the value “How to Figure Out Who’s in Charge.” Any image that does not have a title attribute, or whose title value doesn’t contain the word “Figure,” won’t be matched.

Matching a substring within an attribute value

Sometimes you want to select elements based on a portion of their attribute values, but the values in question aren’t space-separated lists of words. In these cases, you can use the form [att*="val"] to match substrings that appear anywhere inside the attribute values. For example, the following CSS matches any span element whose class attribute contains the substring cloud, so both “cloudy” planets are matched, as shown in Figure 15-12.

span[class*="cloud"] {font-style: italic;}

<span class="barren rocky">Mercury</span>

<span class="cloudy barren">Venus</span>

<span class="life-bearing cloudy">Earth</span>

Figure 15-12. Selecting elements based on substrings within attribute values

As you can imagine, there are many useful applications for this particular capability. For example, suppose you wanted to specially style any links to the O’Reilly Media website. Instead of classing them all and writing styles based on that class, you could simply write the following rule:

a[href*="oreilly.com"] {font-weight: bold;}

Of course, you aren’t confined to the class and href attributes. Any attribute is up for grabs here. title, alt, src, id… you name it, you can style based on a substring within an attribute’s value. The following rule draws attention to any image with the string “space” in its source URL:

img[src*="space"] {border: 5px solid red;}

You aren’t confined to the class and href attributes. Any attribute is up for grabs here. title, alt, src, id… you name it, you can style based on a substring within an attribute’s value. The following rule draws attention to input elements that have a title tells the user what to, and any other input whose title contains the substring “format” in its title:

input[title*="format"] {background-color: #dedede;}

<input type="tel"

title="Telephone number should be formatted as XXX-XXX-XXXX"

pattern="\d{3}\-\d{3}\-\d{4}">

You aren’t confined to the class and href attributes. Any attribute is up for grabs here. title, alt, src, id… you name it, you can style based on a substring within an attribute’s value. A common use for the general substring attribute selector is to match a section of a class in pattern library class names. Elaborating on the example above, we can target any class name that starts with “btn” followed by a dash, that also contains the substring “arrow” preceded by a dash:

*[class|="btn"][class*="-arrow"]:after { content: "▼";}

<button class="btn-small-arrow-active">Click Me</button>

The matches are exact: if you include whitespace in your selector, then whitespace must also be present in an attribute’s value. The attribute names and values must be case-sensitive only if the underlying document language requires case sensitivity. Class names, titles, URLs and id values are all case-sensite, but HTML attribute keyterm values, such as input types are not.

input[type="CHeckBoX"] {margin-right: 10px;}

<input type="checkbox" name="rightmargin" value="10px">

CSS Selectors level 4 will introduce a case-insensitivity flag. Case-sensitivity of attribute values in selectors will continue to depend on whether the attribute was case-sensitive in the document language in general. Including an ibefore the closing bracket will allow the selector to match the attribute values case-insensitively regardless of document language rules.

a[href$='.PDF' i]

When supported, the above will match all links pointing to pdf file formats, regardless of how the PDF was saved.

Matching a substring at the beginning of an attribute value

In cases where you want to select elements based on a substring at the beginning of an attribute value, then the attribute selector pattern [att^="val"] is what you’re seeking. This can be particularly useful in a situation where you want to style types of links differently, as illustrated in Figure 15-13.

a[href^="https:"] {font-weight: bold;}

a[href^="mailto:"] {font-style: italic;}

Figure 15-13. Selecting elements based on substrings that begin attribute values

Another use case is when you want to style all images in an article that are also figures, as in the figures you see throughout this text. Assuming that the alt text of each figure begins with text in the pattern “Figure 5”—which is an entirely reasonable assumption in this case—then you can select only those images as follows:

img[alt^="Figure"] {border: 2px solid gray; display: block; margin: 2em auto;}

The potential drawback here is that any img element whose alt starts with “Figure” will be selected, whether or not it’s meant to be an illustrative figure. The likeliness of that occurring depends on the document in question.

Another use case is selecting all of the calendar events that occur on Mondays. In this case, let’s assume all of the events have a title attribute containing a date in the format “Monday, March 5th, 2012.” Selecting them all is a simple matter of [title^="Monday"].

Matching a substring at the end of an attribute value

The mirror image of beginning-substring matching is ending-substring matching, which is accomplished using the [att$="val"] pattern. A very common use for this capability is to style links based on the kind of resource they target, such as separate styles for PDF documents, as illustrated in Figure 15-14.

a[href$=".pdf"] {font-weight: bold;}

Figure 15-14. Selecting elements based on substrings that end attribute values

Similarly, you could (for whatever reason) select images based on their image format:

img[src$=".gif"] {...}

img[src$=".jpg"] {...}

img[src$=".png"] {...}

To continue the calendar example from the previous section, it would be possible to select all of the events occurring within a given year using a selector like [title$="2015"].

Note

=== You may have noticed that I’ve quoted all the attribute values in the attribute selectors. Quoting is required if the value includes any special characters, begins with a dash or digit, or is otherwise invalid as an identifier and needs to be quoted as a string. To be safe, I recommend always quoting attribute values in attribute selectors, even though it is only required to makes strings out of invalid identifiers. ===

Using Document Structure

As mentioned previously, CSS is powerful because it uses the structure of documents to determine appropriate styles and how to apply them. That’s only part of the story since it implies that such determinations are the only way CSS uses document structure. Structure plays a much larger role in the way styles are applied to a document. Let’s take a moment to discuss structure before moving on to more powerful forms of selection.

Understanding the Parent-Child Relationship

To understand the relationship between selectors and documents, you need to once again examine how documents are structured. Consider this very simple HTML document:

<html>

<head>

<base href="http://www.meerkat.web/">

<title>Meerkat Central</title>

</head>

<body>

<h1>Meerkat <em>Central</em></h1>

<p>

Welcome to Meerkat <em>Central</em>, the <strong>best meerkat web site

on <a href="inet.html">the <em>entire</em> Internet</a></strong>!</p>

<ul>

<li>We offer:

<ul>

<li><strong>Detailed information</strong> on how to adopt a meerkat</li>

<li>Tips for living with a meerkat</li>

<li><em>Fun</em> things to do with a meerkat, including:

<ol>

<li>Playing fetch</li>

<li>Digging for food</li>

<li>Hide and seek</li>

</ol>

</li>

</ul>

</li>

<li>...and so much more!</li>

</ul>

<p>

Questions? <a href="mailto:suricate@meerkat.web">Contact us!</a>

</p>

</body>

</html>

Much of the power of CSS is based on the parent-child relationship of elements. HTML documents (actually, most structured documents of any kind) are based on a hierarchy of elements, which is visible in the “tree” view of the document (see Figure 15-15). In this hierarchy, each element fits somewhere into the overall structure of the document. Every element in the document is either the parent or the child of another element, and it’s often both.

Figure 15-15. A document tree structure

An element is said to be the parent of another element if it appears directly above that element in the document hierarchy. For example, in Figure 15-15, the first p element is parent to the em and strong elements, while strong is parent to an anchor (a) element, which is itself parent to another em element. Conversely, an element is the child of another element if it is directly beneath the other element. Thus, the anchor element in Figure 15-15 is a child of the strong element, which is in turn child to the p element, and so on.

The terms “parent” and “child” are specific applications of the terms ancestor and descendant. There is a difference between them: in the tree view, if an element is exactly one level above another, then they have a parent-child relationship. If the path from one element to another is traced through two or more levels, the elements have an ancestor-descendant relationship, but not a parent-child relationship. (Of course, a child is also a descendant, and a parent is an ancestor.) In Figure 15-15, the first ul element is parent to two li elements, but the first ul is also the ancestor of every element descended from its li element, all the way down to the most deeply nested li elements.

Also, in Figure 15-15, there is an anchor that is a child of strong, but also a descendant of paragraph, body, and html elements. The body element is an ancestor of everything that the browser will display by default, and the htmlelement is ancestor to the entire document. For this reason, in an HTML or XHTML document, the html element is also called the root element.

Descendant Selectors

The first benefit of understanding this model is the ability to define descendant selectors (also known as contextual selectors). Defining descendant selectors is the act of creating rules that operate in certain structural circumstances but not others. As an example, let’s say you want to style only those em elements that are descended from h1 elements. You could put a class attribute on every em element found within an h1, but that’s almost as time-consuming as using the font tag. It’s far more efficient to declare rules that match only em elements that are found inside h1 elements.

To do so, write the following:

h1 em {color: gray;}

This rule will make gray any text in an em element that is the descendant of an h1 element. Other em text, such as that found in a paragraph or a block quote, will not be selected by this rule. Figure 15-16 makes this clear.

Figure 15-16. Selecting an element based on its context

In a descendant selector, the selector side of a rule is composed of two or more space-separated selectors. The space between the selectors is an example of a combinator. Each space combinator can be translated as “found within,” “which is part of,” or “that is a descendant of,” but only if you read the selector right to left. Thus, h1 em can be translated as, “Any em element that is a descendant of an h1 element.” (To read the selector left to right, you might phrase it something like, “Any h1 that contains an em will have the following styles applied to the em.”)

You aren’t limited to two selectors, of course. For example:

ul ol ul em {color: gray;}

In this case, as Figure 15-17 shows, any emphasized text that is part of an unordered list that is part of an ordered list that is itself part of an unordered list (yes, this is correct) will be gray. This is obviously a very specific selection criterion.

Figure 15-17. A very specific descendant selector

Descendant selectors can be extremely powerful. They make possible what could never be done in HTML—at least not without oodles of font tags. Let’s consider a common example. Assume you have a document with a sidebar and a main area. The sidebar has a blue background, the main area has a white background, and both areas include lists of links. You can’t set all links to be blue because they’d be impossible to read in the sidebar.

The solution: descendant selectors. In this case, you give the element (probably a div) that contains your sidebar a class of sidebar, and assign the main area a class of main. Then, you write styles like this:

.sidebar {background: blue;}

.main {background: white;}

.sidebar a:link {color: white;}

.main a:link {color: blue;}

Figure 15-18 shows the result.

Figure 15-18. Using descendant selectors to apply different styles to the same type of element

Note

:link refers to links to resources that haven’t been visited. We’ll talk about it in detail later in this chapter.

Here’s another example: let’s say that you want gray to be the text color of any b (boldface) element that is part of a blockquote, and also for any bold text that is found in a normal paragraph:

blockquote b, p b {color: gray;}

The result is that the text within b elements that are descended from paragraphs or block quotes will be gray.

One overlooked aspect of descendant selectors is that the degree of separation between two elements can be practically infinite. For example, if you write ul em, that syntax will select any em element descended from a ul element, no matter how deeply nested the em may be. Thus, ul em would select the em element in the following markup:

<ul>

<li>List item 1

<ol>

<li>List item 1-1</li>

<li>List item 1-2</li>

<li>List item 1-3

<ol>

<li>List item 1-3-1</li>

<li>List item <em>1-3-2</em></li>

<li>List item 1-3-3</li>

</ol>

</li>

<li>List item 1-4</li>

</ol>

</li>

</ul>

Another, even subtler aspect of descendant selectors is that they have no notion of element proximity. In other words, the closeness of two elements within the document tree has no bearing on whether a rule applies or not. This has bearing when it comes to specificity (which we’ll cover later on) but also when considering rules that might appear to cancel each other out.

For example, consider the following (which contains a selector type we’ll discuss later in this chapter):

div:not(.help) span {color: gray;}

div.help span {color: red;}

<div class="help">

<div class="aside">

This text contains <span>a span element</span> within.

</div>

</div>

What the CSS says, in effect, is “any span inside a div that doesn’t have a class containing the word help should be gray” in the first rule, and “any span inside a div whose class contains the word help” in the second rule. In the given markup fragment, both rules apply to the span shown.

Because the two rules have equal weight and the “red” rule is written last, it wins out and the span is red. The fact that the div class="aside" is “closer to” the span than the div class="help" is completely irrelevant. Again: descendant selectors have no notion of element proximity. Both rules match, only one color can be applied, and due to the way CSS works, red is the winner here.

Selecting Children

In some cases, you don’t want to select an arbitrarily descended element; rather, you want to narrow your range to select an element that is a child of another element. You might, for example, want to select a strong element only if it is a child (as opposed to any level of descendant) of an h1 element. To do this, you use the child combinator, which is the greater-than symbol (>):

h1 > strong {color: red;}

This rule will make red the strong element shown in the first h1 below, but not the second:

<h1>This is <strong>very</strong> important.</h1>

<h1>This is <em>really <strong>very</strong></em> important.</h1>

Read right to left, the selector h1 > strong translates as, “Selects any strong element that is a direct child of an h1 element.” The child combinator can be optionally surrounded by whitespace. Thus, h1 > strong, h1> strong, and h1>strong are all equivalent. You can use or omit whitespace as you wish.

When viewing the document as a tree structure, it’s easy to see that a child selector restricts its matches to elements that are directly connected in the tree. Figure 15-19 shows part of a document tree.

Figure 15-19. A document tree fragment

In this tree fragment, you can easily pick out parent-child relationships. For example, the a element is parent to the strong, but it is child to the p element. You could match elements in this fragment with the selectors p > a and a > strong, but not p > strong, since the strong is a descendant of the p but not its child.

You can also combine descendant and child combinations in the same selector. Thus, table.summary td > p will select any p element that is a child of a td element that is itself descended from a table element that has a classattribute containing the word summary.

Selecting Adjacent Sibling Elements

Let’s say you want to style the paragraph immediately after a heading or give a special margin to a list that immediately follows a paragraph. To select an element that immediately follows another element with the same parent, you use the adjacent-sibling combinator, represented as a plus symbol (+). As with the child combinator, the symbol can be surrounded by whitespace, or not, at the author’s discretion.

To remove the top margin from a paragraph immediately following an h1 element, write:

h1 + p {margin-top: 0;}

The selector is read as, “Selects any p element that immediately follows an h1 element that shares a parent with the p element.”

To visualize how this selector works, it is easiest to once again consider a fragment of a document tree, shown in Figure 15-20.

Figure 15-20. Another document tree fragment

In this fragment, a pair of lists descends from a div element, one ordered and the other not, each containing three list items. Each list is an adjacent sibling, and the list items themselves are also adjacent siblings. However, the list items from the first list are not siblings of the second, since the two sets of list items do not share the same parent element. (At best, they’re cousins, and CSS has no cousin selector.)

Remember that you can select the second of two adjacent siblings only with a single combinator. Thus, if you write li + li {font-weight: bold;}, only the second and third items in each list will be boldfaced. The first list items will be unaffected, as illustrated in Figure 15-21.

Figure 15-21. Selecting adjacent siblings

To work properly, CSS requires that the two elements appear in “source order.” In our example, an ol element is followed by a ul element. This allows you to select the second element with ol + ul, but you cannot select the first using the same syntax. For ul + ol to match, an ordered list must immediately follow an unordered list.

Keep in mind that text content between two elements does not prevent the adjacent-sibling combinator from working. Consider this markup fragment, whose tree view would be the same as that shown in Figure 15-19:

<div>

<ol>

<li>List item 1</li>

<li>List item 1</li>

<li>List item 1</li>

</ol>

This is some text that is part of the 'div'.

<ul>

<li>A list item</li>

<li>Another list item</li>

<li>Yet another list item</li>

</ul>

</div>

Even though there is text between the two lists, you can still match the second list with the selector ol + ul. That’s because the intervening text is not contained with a sibling element, but is instead part of the parent div. If you wrapped that text in a paragraph element, it would then prevent ol + ul from matching the second list. Instead, you might have to write something like ol + p + ul.

As the following example illustrates, the adjacent-sibling combinator can be used in conjunction with other combinators:

html > body table + ul{margin-top: 1.5em;}

The selector translates as, “Selects any ul element that immediately follows a sibling table element that is descended from a body element that is itself a child of an html element.”

As with all combinators, you can place the adjacent-sibling combinator in a more complex setting, such as div#content h1 + div ol. That selector is read as, “Selects any ol element that is descended from a div when the div is the adjacent sibling of an h1 which is itself descended from a div whose id attribute has a value of content.”

Selecting Following Siblings

Selectors Level 3 introduced a new sibling combinator called the general sibling combinator. This lets you select any element that follows another element when both elements share the same parent, represented using the tilde (~) combinator.

As an example, to italicize any ol that follows an h2 and also shares a parent with the h2, you’d write h2 ~ol {font-style: italic;}. The two elements do not have to be adjacent siblings, although they can be adjacent and still match this rule. The result of applying this rule to the following markup is shown in Figure 15-22.

<div>

<h2>Subheadings</h2>

<p>It is the case that not every heading can be a main heading. Some headings must be

subheadings. Examples include:</p>

<ol>

<li>Headings that are less important</li>

<li>Headings that are subsidiary to more important headlines</li>

<li>Headings that like to be dominated</li>

</ol>

<p>Let's restate that for the record:</p>

<ol>

<li>Headings that are less important</li>

<li>Headings that are subsidiary to more important headlines</li>

<li>Headings that like to be dominated</li>

</ol>

</div>

Figure 15-22. Selecting following siblings

As you can see, both ordered lists are italicized. That’s because both of them are ol elements that follow an h2 with whom they share a parent (the div).

Pseudo-Class Selectors

Things get really interesting with pseudo-class selectors. These selectors let you assign styles to what are, in effect, phantom classes that are inferred by the state of certain elements, or markup patterns within the document, or even by the state of the document itself.

The phrase “phantom classes” might seem a little odd, but it really is the best way to think of how pseudo-classes work. For example, suppose you wanted to highlight every other row of a data table. You could do that by marking up every other row something like class="even" and then writing CSS to highlight rows with that class—or (as we’ll soon see) you could use a pseudo-class selector to achieve exactly the same effect, and through very similar means.

Combining Pseudo-Classes

Before we start, a word about chaining. CSS makes it possible to combine (“chain”) pseudo-classes together. For example, you can make unvisited links red when they’re hovered, but visited links maroon when they’re hovered:

a:link:hover {color: red;}

a:visited:hover {color: maroon;}

The order you specify doesn’t actually matter; you could also write a:hover:link to the same effect as a:link:hover. It’s also possible to assign separate hover styles to unvisited and visited links that are in another language—for example, German:

a:link:hover:lang(de) {color: gray;}

a:visited:hover:lang(de) {color: silver;}

Be careful not to combine mutually exclusive pseudo-classes. For example, a link cannot be both visited and unvisited, so a:link:visited doesn’t make any sense.

Structural Pseudo-Classes

Thanks to Selectors Level 3, the majority of pseudo-classes are structural in nature; that is, they refer to the markup structure of the document. Most of them depend on patterns within the markup, such as choosing every third paragraph, but others allow you to address specific types of elements. All pseudo-classes, without exception, are a word preceded by a single colon (:), and they can appear anywhere in a selector.

Before we get started, there’s an aspect of pseudo-classes that needs to be made explicit here: pseudo-classes always refer to the element to which they’re attached, and no other. Seems like a weirdly obvious thing to say, right? The reason to make it explicit is that for a few of the structural pseudo-classes in particular, it’s a common error to think they are descriptors that refer to descendant elements.

To illustrate this, I’d like to share a personal anecdote. When my eldest daughter, also my first child, was born in 2003, I announced it online (like you do). A number of people responded with congratulations and CSS jokes, chief among them the selector #ericmeyer:first-child. The problem there is that selector would select me, and then only if I were the first child of my parents (which, as it happens. I am). To properly select my first child, that selector would need to be #ericmeyer > :first-child.

The confusion is understandable, which is why we’re addressing it here; reminders will be found throughout the following sections. Just always keep in mind that the effect of pseudo-classes is to apply a sort of a “phantom class” to the element to which they’re attached, and you should be okay.

Selecting the root element

This is the quintessence of structural simplicity: the pseudo-class :root selects the root element of the document. In HTML, this is always the html element. The real benefit of this selector is found when writing style sheets for XML languages, where the root element may be different in every language—for example, in RSS 2.0 it’s the rss element—or even when you have more than one possible root element within a single language (though not, of course, a single document!).

Here’s an example of styling the root element in HTML, as illustrated in Figure 15-23:

:root {border: 10px dotted gray;}

body {border: 10px solid black;}

Figure 15-23. Styling the root element

Of course, in HTML documents you can always select the html element directly, without having to use the :root pseudo-class. There is a difference between the two selectors in terms of specificity, which we’ll cover later on.

Selecting empty elements

With the pseudo-class :empty, you can select any element that has no children of any kind, including text nodes, which covers both text and whitespace. This can be useful in suppressing elements that a CMS has generated without filling in any actual content. Thus, p:empty {display: none;} would prevent the display of any empty paragraphs.

Note that in order to be matched, an element must be, from a parsing perspective, truly empty—no whitespace, visible content, or descendant elements. Of the following elements, only the first and last would be matched by p:empty.

<p></p>

<p> </p>

<p>

</p>

<p><!—-a comment--></p>

The second and third paragraphs are not matched by :empty because they are not empty: they contain, respectively, a single space and a single newline character. Both are considered text nodes, and thus prevent a state of emptiness. The last paragraph matches because comments are not considered content, not even whitespace. But put even one space or newline to either side of that comment, and p:empty would fail to match.

Note

CSS Selectors Level 4 will likely include a :blank pseudo-class, that will match the four paragraphs above. :blank is defined to be similar to :empty, in matching empty elements, but to also match elements that contain only white space.

You might be tempted to just style all empty elements with something like *:empty {display: none;}, but there’s a hidden catch: :empty matches HTML’s empty elements, like img and input. It could even match textarea, unless of course you insert some default text into the textarea element. Thus, in terms of matching elements, img and img:empty are effectively the same. (They are different in terms of specificity, which we’ll cover in just a bit.)

<img src="salmon.png" alt="North Pacific Salmon">

<br>

<input type="number" min="-1" max="1" step=".01"/>

<textarea></textarea>

Note

As of early 2015, :empty is unique in that it’s the only CSS selector that takes text nodes into consideration when determining matches. Every other selector type in Selectors Level 3 considers only elements and ignores text nodes entirely—recall, for example, the sibling combinators discussed previously.

Selecting unique children

If you’ve ever wanted to select all the images that are wrapped by a hyperlink element, the :only-child pseudo-class is for you. It selects elements when they are the only child element of another element. So let’s say you want to add a border to any image that’s the only child of another element. You’d write:

img:only-child {border: 1px solid black;}

This would match any image that meets those criteria, of course. Therefore, if you had a paragraph which contained an image and no other child elements, the image would be selected regardless of all the text surrounding it. If what you’re really after is images that are sole children and found inside hyperlinks, then you just modify the selector like so (which is illustrated in Figure 15-24):

a[href] img:only-child {border: 2px solid black;}

<a href="http://w3.org/"><img src="w3.png" alt="W3C"></a>

<a href="http://w3.org/"><img src="w3.png" alt=""> The W3C</a>

<a href="http://w3.org/"><img src="w3.png" alt=""> <em>The W3C</em></a>

<a href="http://w3.org/"><em><img src="w3.png" alt=""> The W3C</em></a>

Figure 15-24. Selecting images that are only children inside links

There are two things to remember about :only-child. The first is that you always apply it to the element you want to be an only child, not to the parent element, as explained earlier. And that brings up the second thing to remember, which is that when you use :only-child in a descendant selector, you aren’t restricting the elements listed to a parent-child relationship. To go back to the hyperlinked-image example, a[href] img:only-child matches any image that is an only child and is descended from an a element, not is a child of an a element. To match, the element image must be the only child of its direct parent, and a descendant of a link, but that parent can itself be a descendant of that link. Therefore all four of the images here would be matched, as shown in Figure 15-25.

a[href] img:only-child {border: 5px solid black;}

<a href="http://w3.org/"><img src="w3.png" alt="W3C"></a>

<a href="http://w3.org/"><span><img src="w3.png" alt="W3C"></span></a>

<a href="http://w3.org/">A link to <span>the <img src="w3.png" alt=""> web</span> site</a>

Figure 15-25. Selecting images that are only children inside links

In each case, the image is the only child element of its parent, and it is also descended from an a element. Thus all three images are matched by the rule shown. If you wanted to restrict the rule so that it matched images that were the only children of a elements, then you’d simply add the child combinator to yield a[href] > img:only-child. With that change, only the first of the three images shown in Figure 15-25 would be matched.

That’s all great, but what if you want to match images that are the only images inside hyperlinks, but there are other elements in there with them? Consider the following:

<a href="http://w3.org/"><b>•</b><img src="w3.png" alt="W3C"></a>

In this case, we have an a element that has two children: a b and an img. That image, no longer being the only child of its parent (the hyperlink), can never be matched using :only-child. However, it can be matched using :only-of-type. This is illustrated in Figure 15-26.

a[href] img:only-of-type {border: 5px solid black;}

<a href="http://w3.org/"><b>•</b><img src="w3.png" alt="W3C"></a>

<a href="http://w3.org/"><span><b>•</b><img src="w3.png" alt="W3C"></span></a>

Figure 15-26. Selecting images that are the only sibling of their type

The difference is that :only-of-type will match any element that is the only of its type among all its siblings, whereas :only-child will only match if an element has no siblings at all.

This can be very useful in cases such as selecting images within paragraphs without having to worry about the presence of hyperlinks or other inline elements:

p > img:only-of-type {float: right; margin: 20px;}

As long as there aren’t multiple images that are children of a paragraph, then the image will be floated. You could also use this pseudo-class to apply extra styles to an h2 when it’s the only one in a section of a document.

section > h2 {margin: 1em 0 0.33em; font-size: 1.8rem; border-bottom: 1px solid gray;}

section > h2:only-of-type {font-size: 2.4rem;}

Given those rules, any section that has only one child h2 will have it appear larger than usual. If there are two or more h2 children to a section, neither of them will be larger than the other. The presence of other children—whether they are other heading levels, paragraphs, tables, paragraphs, lists, and so on—will not interfere with matching.

*.foo:only-of-type {color: red;}

<p class="foo">This paragraph has a class foo</p>

<p>This paragraph doesn't have a class</p>

The :only-of-type pseudoclass matches elements that are the only element of that type. In the above example, the selector will match any element that is the only child of it’s type within a parent selector that also has the class foo. While only one element has the class foo, the p is not the only of its type, and therefore nothing will be matched.

Selecting first and last children

It’s pretty common to want to apply special styling to the first or last child of an element. The most common example is styling a bunch of navigation links into a tab bar, and wanting to put some special visual touches on the first or last tab (or both). In the past, this was done by applying special classes to those elements. Now we have pseudo-classes to carry the load for us.

The pseudo-class :first-child is used to select elements that are the first children of other elements. Consider the following markup:

<div>

<p>These are the necessary steps:</p>

<ul>

<li>Insert key</li>

<li>Turn key <strong>clockwise</strong></li>

<li>Push accelerator</li>

</ul>

<p>

Do <em>not</em> push the brake at the same time as the accelerator.

</p>

</div>

In this example, the elements that are first children are the first p, the first li, and the strong and em elements, which are all the first children of their respective parents. Given the following two rules:

p:first-child {font-weight: bold;}

li:first-child {text-transform: uppercase;}

…you get the result shown in Figure 15-27.

Figure 15-27. Styling first children

The first rule boldfaces any p element that is the first child of another element. The second rule uppercases any li element that is the first child of another element (which, in HTML, must be either an ol or ul element).

As has been mentioned, the most common error is assuming that a selector like p:first-child will select the first child of a p element. However, remember the nature of pseudo-classes, which is to attach a sort of phantom class to the element associated with the pseudo-class. If you were to add actual classes to the markup, it would look like this:

<div>

<p class="first-child">These are the necessary steps:</p>

<ul>

<li class="first-child">Insert key</li>

<li>Turn key <strong class="first-child">clockwise</strong></li>

<li>Push accelerator</li>

</ul>

<p>

Do <em class="first-child">not</em> push the brake at the same time as the

accelerator.

</p>

</div>

Therefore, if you want to select those em elements that are the first child of another element, you write em:first-child.

The mirror image of :first-child is :last-child. If we take the previous example and just change the pseudo-classes, we get the result shown in Figure 15-28.

p:last-child {font-weight: bold;}

li:last-child {text-transform: uppercase;}

<div>

<p>These are the necessary steps:</p>

<ul>

<li>Insert key</li>

<li>Turn key <strong>clockwise</strong></li>

<li>Push accelerator</li>

</ul>

<p>

Do <em>not</em> push the brake at the same time as the accelerator.

</p>

</div>

Figure 15-28. Styling last children

The first rule boldfaces any p element that is the last child of another element. The second rule uppercases any li element that is the last child of another element. If you wanted to select the em element inside that last paragraph, you could use the selector p:last-child em, which selects any em element that descends from a p element that is itself the last child of another element.

Interestingly, you can combine these two pseudo-classes to create a version of :only-child. The following two rules will select the same elements:

p:only-child {color: red;}

p:first-child:last-child {background-color: red;}

Either way, you’d get paragraphs with red foreground and background colors (not a good idea, clearly). The only difference is in terms of specificity, which we’ll cover later in this book.

Selecting first and last of a type

In a manner similar to selecting the first and last children of an element, you can select the first or last of a type of element within an element. This permits things like selecting the first table inside a given element, regardless of whatever other elements come before it.

table:first-of-type {border-top: 2px solid gray;}

Note that this does not apply to the entire document; that is, the rule shown will not select the first table in the document and skip all the others. It will instead select the first table element within each element that contains one, and skip any sibling table elements that come after the first. Thus, given the document structure shown in Figure 15-29, the circled nodes are the ones that are selected.

Figure 15-29. Selecting first-of-type tables

Within the context of tables, a useful way to select the first data cell within a row regardless of whether a header cell comes before it in the row is as follows:

td:first-of-type {border-left: 1px solid red;}

That would select the first data cell in each of the following table rows:

<tr><th scope="row">Count</th><td>7</td><td>6</td><td>11</td></tr>

<tr><td>Q</td><td>X</td><td>-</td></tr>

Compare that to the effects of td:first-child, which would select the first td element in the second row, but not in the first row.

The flip side is :last-of-type, which selects the last instance of a given type from amongst its sibling elements. In a way, it’s just like :first-of-type except you start with the last element in a group of siblings and walk backwards toward the first element until you reach an instance of the type. Given the document structure shown in Figure 15-30, the circled nodes are the ones selected by table:last-of-type.

Figure 15-30. Selecting last-of-type tables

As was noted with :only-of-type, remember that you are selecting elements of a type from among their sibling elements; thus, every set of siblings is considered separately. In other words, you are not selecting the first (or last) of all the elements of a type within the entire document as a single group. Each set of elements that share a parent is its own group, and you can select the first (or last) of a type within each group.

Similar to what was noted in the previous section, you can combine these two pseudo-classes to create a version of :only-of-type. The following two rules will select the same elements:

table:only-of-type{color: red;}

table:first-of-type:last-of-type {background: red;}

Selecting every nth child

If you can select elements that are the first, last, or only children of other elements, how about every third child? All even children? Only the ninth child? Rather than define a literally infinite number of named pseudo-classes, CSS has the :nth-child() pseudo-class. By filling integers or even simple algebraic expressions into the parentheses, you can select any arbitrarily numbered child element you like.

Let’s start with the :nth-child() equivalent of :first-child, which is :nth-child(1). In the following example, the selected elements will be the first paragraph and the first list item.

p:nth-child(1) {font-weight: bold;}

li:nth-child(1) {text-transform: uppercase;}

<div>

<p>These are the necessary steps:</p>

<ul>

<li>Insert key</li>

<li>Turn key <strong>clockwise</strong></li>

<li>Push accelerator</li>

</ul>

<p>

Do <em>not</em> push the brake at the same time as the accelerator.

</p>

</div>

If we were to change the numbers from 1 to 2, however, then no paragraphs would be selected, and the middle (or second) list item would be selected, as illustrated in Figure 15-31.

p:nth-child(2) {font-weight: bold;}

li:nth-child(2) {text-transform: uppercase;}

Figure 15-31. Styling second children

You can of course insert any integer you choose; if you have a use case for selecting any ordered list that is the 93rd child element of its parent, then ol:nth-child(93) is ready to serve. This will match the 93rd child of any parent as long as that child is an ordered list. (This does not mean the 93rd ordered list among its siblings; see the next section for that.)

More powerfully, you can use simple algebraic expressions in the form a n + b or a n − b to define recurring instances, where a and b are integers and n is present as itself. Furthermore, the + b or − b part is optional and thus can be dropped if it isn’t needed.

Let’s suppose we want to select every third list item in an unordered list, starting with the first. The following makes that possible, selecting the first and fourth items, as shown in Figure 15-32.

ul > li:nth-child(3n + 1) {text-transform: uppercase;}

Figure 15-32. Styling every third list item

The way this works is that n represents the series 0, 1, 2, 3, 4, …and on into infinity. The browser then solves for 3_n_ + 1, yielding 1, 4, 7, 10, 13, …and so on. Were we to drop the +1, thus leaving us with simply 3n, the results would be 0, 3, 6, 9, 12, …and so on. Since there is no zeroth list item—all element counting starts with one, to the likely chagrin of array-slingers everywhere—the first list item selected by this expression would be the third list item in the list.

Given that element counting starts with one, it’s a minor trick to deduce that :nth-child(2n) will select even-numbered children, and either :nth-child(2n+1) or :nth-child(2n-1) will select odd-numbered children. You can commit that to memory, or you can use the two special keywords that :nth-child() accepts: even and odd. Want to highlight every other row of a table, starting with the first? Here’s how you do it, as shown in Figure 15-33.

tr:nth-child(odd) {background: silver;}

Figure 15-33. Styling every other table row

Anything more complex than every-other-element requires an an + b expression.

Note that when you want to use a negative number for b, you have to remove the + sign or else the selector will fail entirely. Of the following two rules, only the first will do anything. The second will be dropped by the parser and ignored.

tr:nth-child(4n - 2) {background: silver;}

tr:nth-child(3n + −2) {background: red;}

If you wanted to select every row starting with the 9th, you can use either of the following. They are similar in that they will select all rows starting with the 9th, but the latter one has greater specificity, which we discuss later.

tr:nth-child(n + 9) {background: silver;}

tr:nth-child(8) ~ tr {background: silver;}

As you might expect, there is a corresponding pseudo-class in :nth-last-child(). This lets you do the same thing as :nth-child(), except with :nth-last-child() you start from the last element in a list of siblings and count backwards toward the beginning. If you’re intent on highlighting every other table row and making sure the very last row is one of the rows in the highlighting pattern, either one of these will work for you:

tr:nth-last-child(odd) {background: silver;}

tr:nth-last-child(2n+1) {background: silver;} /* equivalent */

If the DOM is updated to add or remove table rows, there is no need to add or remove classes. By using structural selectors, these selectors will always match the odd rows of the updated DOM.

Of course, any element can be matched using both :nth-child() and :nth-last-child() if it fits the criteria. Consider these rules, the results of which are shown in Figure 15-34.

li:nth-child(3n + 3) {border-left: 5px solid black;}

li:nth-last-child(4n - 1) {border-right: 5px solid black;}

Figure 15-34. Combining patterns of :nth-child() and :nth-last-child()

It’s also the case that you can string these two pseudo-classes together as :nth-child(1):nth-last-child(1), thus creating a more verbose restatement of :only-child. There’s no real reason to do so other than to create a selector with a higher specificity, but the option is there.

You can use CSS to determine how many list-items are in a list, and style them accordingly.

li:only-child {width: 100%;}

li:nth-child(1):nth-last-child(2),

li:nth-child(2):nth-last-child(1) {width: 50%;}

li:nth-child(1):nth-last-child(3),

li:nth-child(1):nth-last-child(3) ~ li {width: 33.33%;}

li:nth-child(1):nth-last-child(4),

li:nth-child(1):nth-last-child(4) ~ li {width: 25%;}

You can use CSS to determine how many list-items are in a list, and style them accordingly. In the above examples, if a list-item is the only list item, then the width is 100%. If a list item is the first item and also the second from last item, that means there are 2 items, and we made the width 50%. If an item is the first item and also the third from the last item, then we made it, and the two sibling list items following it, 33% wide. Similarly, if a list item is the first item and also the fourth from the last item, it means that there are exactly four items, so we made it, and its 3 siblings, 25% of the width.

Selecting every nth of a type

In what’s no doubt become a very familiar pattern, the :nth-child() and :nth-last-child() pseudo-classes have analogues in :nth-of-type() and :nth-last-of-type(). You can, for example, select every other hyperlink that’s a child of any given paragraph, starting with the second, using p > a:nth-of-type(even). This will ignore all other elements (spans, strongs, etc.) and consider only the links, as demonstrated in Figure 15-35:

p > a:nth-of-type(even) {background: blue; color: white;}

Figure 15-35. Selecting the even-numbered links

If you wanted to work from the last hyperlink backwards, then of course you’d use p > a:nth-last-of-type(even).

As before, these select elements of a type from among their sibling elements, not from among all the elements of a type within the entire document as a single group. Each element has its own list of siblings, and selections happen within each group.

As you might expect, you can string these two together as :nth-of-type(1):nth-last-of-type(1) to restate :only-of-type, only with higher specificity. (We will explain specificity later in this book, I promise.)

Dynamic Pseudo-Classes

Beyond the structural pseudo-classes, there are a set of pseudo-classes that relate to structure but can change based on changes made to the page after it’s been rendered. In other words, the styles are applied to pieces of a document based on something in addition to the structure of the document, and in a way that cannot be precisely deduced simply by studying the document’s markup.

It may sound like we’re applying styles at random, but not so. Instead, we’re applying styles based on somewhat ephemeral conditions that can’t be predicted in advance. Nevertheless, the circumstances under which the styles will appear are, in fact, well-defined. Think of it this way: during a sporting event, whenever the home team scores, the crowd will cheer. You don’t know exactly when during a game the team will score, but when it does, the crowd will cheer, just as predicted. The fact that you can’t predict the moment of the cause doesn’t make the effect any less expected.

Consider the anchor element (a), which (in HTML and related languages) establishes a link from one document to another. Anchors are always anchors, of course, but some anchors refer to pages that have already been visited, while others refer to pages that have yet to be visited. You can’t tell the difference by simply looking at the HTML markup, because in the markup, all anchors look the same. The only way to tell which links have been visited is by comparing the links in a document to the user’s browser history. So there are actually two basic types of anchors: visited and unvisited.

Hyperlink pseudo-classes

CSS2.1 defines two pseudo-classes that apply only to hyperlinks. In HTML, these are any a elements with an href attribute; in XML languages, they’re any elements that act as links to another resource. Table 15-2 describes these two pseudo-classes.

The first of the pseudo-classes in Table 15-2 may seem a bit redundant. After all, if an anchor hasn’t been visited, then it must be unvisited, right? If that’s the case, all we should need is the following:

a {color: blue;}

a:visited {color: red;}

Although this format seems reasonable, it’s actually not quite enough. The first of the rules shown here applies not only to unvisited links, but also to placeholder links such as this one:

<a>4. The Lives of Meerkats</a>

The resulting text would be blue because the a element will match the rule a {color: blue;}, as shown above. Therefore, to avoid applying your link styles to placeholders, use the :link and :visited pseudo-classes:

a:link {color: blue;} /* unvisited links are blue */

a:visited {color: red;} /* visited links are red */

As you may or may not have already realized, the :link and :visited pseudo-class selectors are functionally equivalent to the early-1990s body attributes link and vlink. Assume that an author wants all anchors to unvisited pages to be purple and anchors to visited pages to be silver. Back in the days of HTML 3.2, this could be specified as follows:

<body link="purple" vlink="silver">

Don’t do this. Instead, use CSS. In CSS, the effect is accomplished with:

a:link {color: purple;}

a:visited {color: silver;}

This is a good place to revisit attribute and class selectors and show how they can be combined with pseudo-classes. For example, let’s say you want to change the color of links that point outside your own site. In most circumstances we can use the starts-with attribute selector. However, some CMS’s set all links to be absolute URLS, in which case you could assign a class to each of these anchors, it’s easy:

<a href="/about.html">My About page</a>

<a href="https://www.site.net/" class="external">An external site</a>

To apply different styles to the external link, all you need is a rule like this:

a.external:link, a[href^="http"]:link { color: slateblue;}

a.external:visited, a[href^="http"]:visited {color: maroon;}

This rule will make the second anchor in the preceding markup slateblue by default, and maroon once visited, while the first anchor will remain the default color for hyperlinks (usually blue when not-visited and purple once visited). For improved usability and accessibility, visited links should be easily differentiable from non-visited links.

Note

Styled visited links enables visitors to know where they have been and what they have yet to visit. This is especially important on large websites where it may be difficult to remember, especially for those with cognitive disabilities, which pages have been visited. Not only is highlighting visited links one of the W3C Web Content Accessibility Guidelines, but it makes searching for content faster, more efficient and less stressful for everyone.

The same general syntax is used for ID selectors as well:

a#footer-copyright:link{background: yellow;}

a#footer-copyright:visited {background: gray;}

You can chain the two link-state pseudo-classes together, but there’s no reason why you ever would: a link cannot be both visited and unvisited at the same time!

Visited links and privacy

For well over a decade, it was possible to style visited links with any CSS properties available, just as you could unvisited links. However, in the mid-2000s several people demonstrated that one could use visual styling and simple DOM scripting to determine if a user had visited a given page. For example, given the rule :visited {font-weight: bold;} a script could find all of the boldfaced links and tell the user which of those sites they’d visited—or, worse still, report those sites back to a server. A similar, non-scripted tactic uses background images to achieve the same result.

While this might not seem terribly serious to you, it can be utterly devastating for a web user in a country where one can be jailed for visiting certain sites—opposition parties, unsanctioned religious organizations, “immoral” or “corrupting” sites, and so on. It can also be used by phishing sites to determine which online banks a user has visited. Thus, two steps were taken.

The first step is that only color-related properties can be applied to visited links: color, background-color, column-rule-color, outline-color, border-color, and the individual-side border color properties (e.g., border-top-color). Attempts to apply any other property to a visited link will be ignored. Furthermore, any styles defined for :link will be applied to visited links as well as unvisited links, which effectively makes :link “style any hyperlink,” instead of “style any unvisited hyperlink.”

The second step is that if a visited link has its styles queried via the DOM, the resulting value will be as if the link were not visited. Thus, if you’ve defined visited links to be purple rather than unvisited links’ blue, even though the link will appear purple onscreen, a DOM query of its color will return the blue value, not the purple.

As of mid-2015, this behavior is present throughout all browsing modes, not just “private browsing” modes. Even though we’re limited in how we can use CSS to differentiate visited links from non-visited links, it is important for usability and accessibility to use the limited styles supported by :visited links to differentiate them from unvisited links.

User action pseudo-classes

CSS defines a few pseudo-classes that can change a document’s appearance based on actions taken by the user. These dynamic pseudo-classes have traditionally been used to style hyperlinks, but the possibilities are much wider. Table 15-3 describes these pseudo-classes.

Elements that can become :active include links, buttons, menuitems and any element with a tabindex value. These elements all all other interactive elements, including form controls and elements that are contenteditable, can also receive focus.

As with :link and :visited, these pseudo-classes are most familiar in the context of hyperlinks. Many web pages have styles that look like this:

a:link {color: navy;}

a:visited {color: gray;}

a:focus {color: orange;}

a:hover {color: red;}

a:active {color: yellow;}

Note

The order of the pseudo-classes is more important than it might seem at first. The usual recommendation is “link-visited-hover-active,” although this has been modified to “link-visited-focus-hover-active.” The next chapter explains why this particular ordering is important and discusses several reasons you might choose to change or even ignore the recommended ordering.

Notice that the dynamic pseudo-classes can be applied to any element, which is good since it’s often useful to apply dynamic styles to elements that aren’t links. For example, using this markup:

input:focus {background: silver; font-weight: bold;}

…you could highlight a form element that is ready to accept keyboard input, as shown in Figure 15-36.

Figure 15-36. Highlighting a form element that has focus

Note

While you can style element with :focus anyway you like, do not remove all styling from focused elements, as differentiating which element currently has focus is vital for accessibility, especially for those navigating your site or application with a keyboard.

You can also perform some rather odd feats by applying dynamic pseudo-classes to arbitrary elements. You might decide to give users a “highlight” effect by way of the following:

body *:hover {background: yellow;}

This rule will cause any element that’s descended from the body element to display a yellow background when it’s in a hover state. Headings, paragraphs, lists, tables, images, and anything else found inside the body will be changed to have a yellow background. You could also change the font, put a border around the element being hovered, or alter anything else the browser will allow.

Real-world issues with dynamic styling

Dynamic pseudo-classes present some interesting issues and peculiarities. For example, it’s possible to set visited and unvisited links to one font size and make hovered links a larger size, as shown in Figure 15-37:

a:link, a:visited {font-size: 13px;}

a:hover, a:active {font-size: 20px;}

Figure 15-37. Changing layout with dynamic pseudo-classes

As you can see, the user agent increases the size of the anchor while the mouse pointer hovers over it, or, thanks to the :active setting, when a user touches it on a touch screen. A user agent that supports this behavior must redraw the document while an anchor is in hover state, which could force a reflow of all the content that follows the link.

UI State Pseudo-Classes

Closely related to the dynamic pseudo-classes are the user interface (UI) state pseudo-classes, which are summarized in Table 15-4. These pseudo-classes allow for styling based on the current state of user interface elements like checkboxes.

Although the state of a UI element can certainly be changed by user action—e.g., a user checking or unchecking a checkbox—UI state pseudo-classes are not classified as purely dynamic because they can also be affected by the document structure or DOM scripting.

Note

You might think that :focus belongs in this section, not the previous section. However, the Selectors Level 3 specification groups :focus in with :hover and :active. This is most likely because they were grouped together in CSS2, which had no UI state pseudo-classes. More importantly, though, focus can be given to non-UI elements, such as headings or paragraphs—one example is when they are read by a speaking browser. That alone keeps it from being considered a UI-state pseudo-class.

Enabled and disabled UI elements

Thanks to both DOM scripting and HTML5, it is possible to mark a user interface element (or group of user interface elements) as being disabled. A disabled element is displayed, but cannot be selected, activated, or otherwise interacted with by the user. Authors can set an element to be disabled either through DOM scripting, or (in HTML5) by adding a disabled attribute to the element’s markup.

Any element that hasn’t been disabled is by definition enabled, and you can style these two states using :enabled and :disabled. It’s much more common to simply style disabled elements and leave enabled elements alone, but both have their uses, as illustrated in Figure 15-38.

:enabled {font-weight: bold;}

:disabled {opacity: 0.5;}

Figure 15-38. Styling enabled and disabled UI elements

Check states

In addition to being enabled or disabled, certain UI elements can be checked or unchecked—in HTML, the input types “checkbox” and “radio” fit this definition. Selectors level 3 offers a :checked pseudo-class to handle elements in that state, though curiously it omits an :unchecked. There is also the :indeterminate pseudo-class, which matches any checkable UI element that is neither checked nor unchecked. These states are illustrated in Figure 15-39.

:checked {background: silver;}

:indeterminate {border: red;}

In addition, you can use the negation pseudo-class, which is covered later, to select checkboxes which are not checked with input[type="checkbox]:not(:checked). Only radio buttons and checkboxes can be checked. All other elements, and these two when not checked, are :not(:checked)

Figure 15-39. Styling checked and indeterminate UI elements

Although checkable elements are unchecked by default, it’s possible for a HTML author to toggle them on by adding the checked attribute to an element’s markup. An author can also use DOM scripting to flip an element’s checked state to checked or unchecked, whichever they prefer.

As we’ve seen, there is a third state, “indeterminate.” As of mid-2015, this state can only be set through DOM scripting; there is no markup-level method to set elements to an indeterminate state. The purpose of allowing an indeterminate state is to visually indicate that the element needs to be checked (or unchecked) by the user. However, note that this is purely a visual effect: it does not affect the underlying state of the UI element, which is either checked or unchecked, depending on document markup and the effects of any DOM scripting.

Note

Although Figure 15-39 does show styled radio buttons, styling of radio buttons and checkboxes with CSS is very limited. There aren’t many CSS properties that apply to checkboxes and radio buttons, but that shouldn’t limit your use of the selected-option pseudo-classes. Used in combination with the adjacent sibling selector, you can style the labels associated with your checkboxes and radio buttons:

<input id="chbx" type="checkbox"> <label for="chbx">I am a label</label>

input[type="checkbox"]:checked + label {

color: red;

font-style: italic;

}

Default Option pseudo-class

The :default pseudo-class matches the UI elements that are the default among a set of similar elements. Typically applies to context menu items, buttons and select lists/menus. If there are several same-named radio buttons, the one that was originally checked matches :default, even if the UI has been updated by the user so that it no longer matches :checked. If a checkbox was checked on page load, it would match. Any initially-selected <option>(s) in a <select> will match. The :default pseudo-class can also match buttons and menu items.

<input type="checkbox" id="chbx" checked name="foo" value="bar"> <label for="chbx">This was checked on page load</label>

[type="checkbox"]:default + label { font-style: italic; }

Validity Pseudo-Classes

With the introduction of HTML5 web forms, there are new input types, many new attributes for the <input> element, and native validation. There are several newer UI pseudo-classes that match elements based on the current state of the user interface. Now form controls can be required to match a specific regular expression, can be of a specific data type. Numeric input types, including date and time input types, can be required to have a value, with the value being between a prescribed minimum and maximum value, and ever a value of a specific increment over the minimum value. The UI pseudo-classes target these form controls based on whether the value is currently valid or not, according to the constraints set by the element’s attributes.

Optionality Pseudo-classes

The optionality pseudo-class of :required matches any form control that is required, as denoted by the presence of the required attribute. The :optional pseudo-class matches form controls that do not have the required attribute, or whose required attribute is set to fale.

A form element is :required or :optional if a value for it is, respectively, required or optional before the form it belongs to can be validly submitted. Elements that are not form elements are neither required nor optional.

The :required pseudo-class refers to a user-input that must have a value set. The :optional pseudo-class refers to a user-input that does not need to have a value set.

<input type="email" placeholder="enter an email address" required>

<input type="email" placeholder="optional email address">

input:required { border: 1px solid #f00;}

input:optional { border: 1px solid #ccc;}

The first input will match the :required pseudo-class because of the presence of the required attribute. The second input is optional, and therefore will match the :optional pseudo-class.

We could have used attribute selectors instead. The selectors below are equivalent to the ones above:

input[required] { border: 1px solid #f00;}

input:not([required]) { border: 1px solid #ccc;}

Validity Pseudo-Classes

The :valid pseudo-class refers to a user-input that meets all of its data validity semantics. The :invalid pseudo-class, on the other hand, refers to a user-input that does not meet all of its data validity semantics. The validity pseudo-classes: :valid and :invalid, only apply to elements having the capacity for data validity semantics: a <div> will never match either selector, but an <input> could match either, depending on the current state of the UI, even if no constraints were attached to that particular input.

<input type="tel" pattern="\d{3}\-\d{3}\-\d{4}" required>

input:focus:required {

background-position: 100% 50%;

background-repeat: no-repeat;

}

input:focus:required:invalid {

background-image: url(warning.jpg);

}

input:focus:required:valid {

background-image: url(checkmark.jpg);

}

In the above example, we style only required input types when they have focus with a warning icon when the value is not a valid value, and a checkmark when it is. By combining pseudo-classes, we improved usability: the document would be too busy with icons showing all the time. We only include icons when the user is interacting with the form control, letting them know via progressive enhancement when their value is valid.

Range Pseudo-classes:

The range Pseudo-classes include :in-range, which refers to a user-input whose value is between the minimum and maximum values set by HTML5’s min and max attributes, and :out-of-range+, which refers to a user-input whose value is below the minimum or above the maximum values allowed by the control.

<input id="nickels" type="number" min="0" max="1000" step="5" />

input:focus:out-of-range {

background-image: url(warning.jpg);

}

input:focus:in-range {

background-image: url(checkmark.jpg);

}

The :in-range and :out-of-range pseudo-classes apply only to elements with range limitations. Elements that don’t have range limitations, like links for inputs of type tel, will match neither neither pseudo-class.

There is also a step attribute in HTML5. If they value is invalid because it does not match the step, but is still between or equal to the min and max values, it will match :invalid, but it is still :in-range.

Mutability Pseudo-classes

The mutability pseudo-classes include :read-write, which refers to a user-input that is editable by the user, and :read-only, matching user-inputs that are not editable. Only elements that have the capacity to be alterable can match :read-write. For example, in HTML, a non-disabled, non-readonly <input> element is :read-write, as is any element with the +contenteditable attribute. Everything else, including the body, as long as it is not contenteditable, matches :read-only.

input:read-only { opacity: 0.75;}

pre:read-write:hover {border: 1px dashed green;}

The :target Pseudo-Class

When a URL includes a fragment identifier, the piece of the document at which it points is called (in CSS) the target. Thus you can uniquely style any element that is the target of a URL fragment identifier with the :target pseudo-class.

Even if you’re unfamiliar with the term “fragment identifier,” you’ve almost certainly seen them in action. Consider this URL:

http://www.w3.org/TR/css3-selectors/#target-pseudo

The target-pseudo portion of the URL is the fragment identifier, which is marked by the # symbol. If the referenced page (http://www.w3.org/TR/css3-selectors/) has an element with an ID of target-pseudo, then that element becomes the target of the fragment identifier.

Thanks to :target, you can highlight any targeted element within a document, or you can devise different styles for various types of elements that might be targeted—say, one style for targeted headings, another for targeted tables, and so on. Figure 15-40 shows an example of :target in action:

*:target {border-left: 5px solid gray; background: yellow url(target.png)

top right no-repeat;}

Figure 15-40. Styling a fragment identifier target

Somewhat obviously, :target styles will not be applied in two circumstances:

1. If the page is accessed via a URL that does not have a fragment identifier.

2. If the page is accessed via a URL that has a fragment identifier, but the identifier does not match any elements within the document.

More interestingly, though, what happens if multiple elements within a document can be matched by the fragment identifier—for example, if the author erroneously included three separate instances of <div id="target-pseudo"> in the same document?

The short answer is that CSS doesn’t have or need rules to cover this case, because all CSS is concerned with is styling targets. Whether the browser picks just one of the three elements to be the target or designates all three as co-equal targets, :target styles should be applied to anything that is a target.

The :lang Pseudo-Class

For situations where you want to select an element based on its language, you can use the :lang() pseudo-class. In terms of its matching patterns, the :lang() pseudo-class is similar to the |= attribute selector. For example, to italicize elements whose content is written in French, you could write either of the following:

*:lang(fr) {font-style: italic;}

*[lang|="fr"] {font-style: italic;}

The primary difference between the pseudo-class selector and the attribute selector is that language information can be derived from a number of sources, some of which are outside the element itself. For the attribute selector, the element must have hte attribute present to match. The :lang pseudo-class, however, matches descendants of an element with the language declaration. As Selectors Level 3 states:

…in HTML, the language is determined by a combination of the lang attribute, and possibly information from the meta elements and the protocol (such as HTTP headers). XML uses an attribute called xml:lang, and there may be other document language-specific methods for determining the language.

The pseudo-class will operate on all of that information, whereas the attribute selector can only work if there is a lang attribute present in the element’s markup. Therefore, the pseudo-class is more robust than the attribute selector and is probably a better choice in most cases where language-specific styling is needed.

The Negation Pseudo-Class

Every selector we’ve covered thus far has had one thing in common: they’re all positive selectors. In other words, they are used to identify the things that should be selected, thus excluding by implication all the things that don’t match and are thus not selected.

For those times when you want to invert this formulation and select elements based on what they are not, Selectors Level 3 introduced the negation pseudo-class, :not(). It’s not quite like any other selector, fittingly enough, and it does have some restrictions on its use, but let’s start with an example.

Let’s suppose you want to apply a style to every list item that doesn’t have a class of moreinfo, as illustrated in Figure 15-41. That used to be very difficult, and in certain cases impossible, to make happen. If we wanted all the list items to be italic except those with the class .moreinfo, we used to declare all the links as italic, generally having to target the ul with a class, then normalize back based on the class, making sure the override came last in the source order, and had equal or higher specificity. Now we can simply declare:

li:not(.moreinfo) {font-style: italic;}

Figure 15-41. Styling list items that don’t have a certain class

The way :not() works is that you attach it to an element, and then in the parentheses you fill in a simple selector. A simple selector, according to the W3C, is:

…either a type selector, universal selector, attribute selector, class selector, ID selector, or pseudo-class.

Basically, a simple selector is a selector with no ancestral / descendant relationship.

Note the “either” there: you can only use one of those inside :not(). You can’t group them and you can’t combine them using combinators, which means you can’t use a descendant selector, because the space separating elements in a descendant selector is a combinator. Those restrictions may (indeed most likely will) be lifted in a post-Level 3 specification, but we can still do quite a lot even within the given constraints.

For example, let’s flip around the previous example and select all elements with a class of moreinfo that are not list items. This is illustrated in Figure 15-42.

.moreinfo:not(li) {font-style: italic;}

Figure 15-42. Styling elements with a certain class that aren’t list items

Translated into English, the selector would say, “Select all elements with a class whose value contains the word moreinfo as long as they are not li elements.” Similarly, the translation of li:not(.moreinfo) would be “Select all lielements as long as they do not have a class whose value contains the word moreinfo.”

Technically, you can put a universal selector into the parentheses, but there’s very little point. After all, p:not(*) would mean “Select any p element as long as it isn’t any element” and there’s no such thing as an element that is not an element. Very similar to that would be p:not(p), which would also select nothing. It’s also possible to write things like p:not(div), which will select any p element that is not a div element—in other words, all of them. Again, there is very little reason to do so.

You can also use the negation pseudo-class at any point in a more complex selector. Thus to select all tables that are not children of a section element, you would write *:not(section) > table. Similarly, to select table header cells that are not part of the table header, you’d write something like table *:not(thead) > tr > th, with a result like that shown in Figure 15-43.

Figure 15-43. Styling table cells that aren’t in the table’s header

What you cannot do is nest negation pseudo-classes; thus, p:not(:not(p)) is invalid and will be ignored. It’s also, logically, the equivalent of simply writing p, so there’s no point anyway. Furthermore, you cannot reference pseudo-elements (which we’ll cover shortly) inside the parentheses, since they are not simple selectors.

On the other hand, it’s possible to chain negations together to create a sort of “and also not this” effect. For example, you might want to select all elements with a class of link that are neither list items nor paragraphs.

*.link:not(li):not(p) {font-style: italic;}

That translates to “Select all elements with a class whose value contains the word link as long as they are neither li nor p elements.”

One thing to watch out for is that you can have situations where rules combine in unexpected ways, mostly because we’re not used to thinking of selection in the negative. Consider this very simple test case:

div:not(.one) p {font-weight: normal;}

div.one p {font-weight: bold;}

<div class="one">

<div class="two">

<p>I'm a paragraph!</p>

</div>

</div>

The paragraph will be boldfaced, not normal-weight. This is because both rules match: the p element is descended from a div whose class does not contain the word one (<div class="two">), but it is also descended from a divwhose class contains the word one. Both rules match, and so both apply. Since there is a conflict, the cascade is used to resolve the conflict, and the second rule wins. The structural arrangement of the markup, with the div.two being “closer” to the paragraph than div.one, is irrelevant.

Note

An interesting aspect of the negation pseudo-class is that it, like the universal selector, does not count towards specificity, but the selector passed as the parameter does. We cover specificity later in the book.

Pseudo-Element Selectors

Much as pseudo-classes assign phantom classes to anchors, pseudo-elements insert fictional elements into a document in order to achieve certain effects. Four pseudo-elements are defined in CSS, and they let you style the first letter of an element, style the first line of an element, and both create and style “before” and “after” content.

Unlike the single colon of pseudo-classes, pseudo-elements employ a double-colon syntax, like ::first-line. This is meant to distinguish pseudo-elements from pseudo-classes. This was not always the case—in CSS2, both selector types used a single colon—so for backwards compatibility, browsers will accept single-colon pseudo-element selectors. Don’t take this as an excuse to be sloppy, though! Use the proper number of colons at all times in order to future-proof your CSS; after all, there is no way to predict when browsers will stop accepting single-colon pseudo-element selectors.

Note that all pseudo-elements must be placed at the very end of the selector in which they appear. Therefore, it would not be legal to write p::first-line em since the pseudo-element comes before the subject of the selector (the subject is the last element listed). This also means that only one pseudo-element is permitted in a given selector, though that restriction may be eased in future versions of CSS.

Styling the First Letter

The first pseudo-element styles the first letter, or punctuation characters and the first letter if the text starts with punctuation, of any non-inline element.

p::first-letter {color: red;}

This rule causes the first letter of every paragraph to be colored red. The ::first-letter pseudo-element is most commonly used to create an “initial cap” or “drop cap” typographic effect. You could make the first letter of each p twice as big as the rest of the heading, though you may want to only apply this styling to the first letter of the first paragraph:

p:first-of-type::first-letter {font-size: 200%;}

The result of this rule is illustrated in Figure 15-44.

Figure 15-44. The ::first-letter pseudo-element in action

As mentioned, this rule effectively causes the user agent to respond to a fictional, or “faux” element, that encloses the first letter of each p. It would look something like this:

<p><p-first-letter>T</hp-first-letter>his is a p element, with a styled first letter</h2>

The ::first-letter styles are applied only to the contents of the fictional element shown in the example. This <p-first-letter> element does not appear in the document source, nor even in the DOM tree. Instead, its existence is constructed on the fly by the user agent and is used to apply the ::first-letter style(s) to the appropriate bit of text. In other words, <p-first-letter> is a pseudo-element. Remember, you don’t have to add any new tags. The user agent style the first letter for you as if you had encased it in a styled element.

The first-letter is defined as the first typographic letter unit of the originating element, if it is not preceded by other, like an image. The specifications use “letter unit” because some languages have letters made up of more than character, like “oe” in Norse. Punctuation that precede or follow the first letter unit, even if there are several of them, are included in the ::first-letter pseudo-element.

Styling the First Line

Similarly, ::first-line can be used to affect the first line of text in an element. For example, you could make the first line of each paragraph in a document large and purple:

p::first-line {

font-size: 150%;

color: purple;

}

In Figure 15-45, the style is applied to the first displayed line of text in each paragraph. This is true no matter how wide or narrow the display region is. If the first line contains only the first five words of the paragraph, then only those five words will be big and purple. If the first line contains the first 30 words of the element, then all 30 will be big and purple.

Figure 15-45. The ::first-line pseudo-element in action

Because the text from “This” to “only” should be big and purple, the user agent employs a fictional markup that looks something like this:

<p><p-first-line>This is a paragraph of text that has only</p-first-line>

one stylesheet applied to it. That style

causes the first line to be big and purple.

No other line will inherit the styles from...

If the first line of text were edited to include only the first seven words of the paragraph, then the fictional </p-first-line> would move back and occur just after the word “that.” If the user were to increase or decrease the font-size rendering, or expand or contract the browser window causing the width of the text to change, thereby causing the number of words on the first line to increase or decrease, the browser automatically sets only the words in the currently displayed first line to be both big and purple.

The length of the first line depends on a number of factors, including the font-size, letter spacing, width of the parent container, etc. Depending on the markup, and the length of that first line, it is possible that the end of the first line comes in the middle of a nested element. If the ::first-line breaks up a nested element, such as an em or a link, the properties attached to the ::first-line will only apply to the portion of that nested element that is displayed on the first line.

Restrictions on ::first-letter and ::first-line

The ::first-letter and ::first-line pseudo-elements currenly can be applied only to block-display elements such as headings or paragraphs and not to inline-display elements such as hyperlinks. There are also limits on the CSS properties that may be applied to ::first-line and ::first-letter. Table 15-5 displays the limits.

The faux ::first-line and first-letter pseudo-elements that are implemented but not really created, are similar to an inline-level span around that first line and first letter respectively that can be styles, but have limited CSS properties that can be applied to them.

Styling (Or Creating) Content Before and After Elements

Let’s say you want to preface every h2 element with a pair of silver square brackets as a typographical effect:

h2::before {content: "]]"; color: silver;}

CSS2.1 lets you insert generated content, and then style it directly using the pseudo-elements ::before and ::after. Figure 15-46 illustrates an example.

Figure 15-46. Inserting content before an element

The pseudo-element is used to insert the generated content and to style it. To place content after an element, use the pseudo-element ::after. You could end your documents with an appropriate finish:

body::after {content: "The End.";}

Generated content is a separate subject, and the entire topic (including more detail on ::before and ::after) is covered more thoroughly in just a bit.

Summary

By using selectors based on the document’s language, authors can create CSS rules that apply to a large number of similar elements just as easily as they can construct rules that apply in very narrow circumstances. The ability to group together both selectors and rules keeps style sheets compact and flexible, which incidentally leads to smaller file sizes and faster download times.

Selectors are the one thing that user agents usually must get right because the inability to correctly interpret selectors pretty much prevents a user agent from using CSS at all. On the flip side, it’s crucial for authors to correctly write selectors because errors can prevent the user agent from applying the styles as intended. An integral part of correctly understanding selectors and how they can be combined is a strong grasp of how selectors relate to document structure and how mechanisms—such as inheritance and the cascade itself—come into play when determining how an element will be styled.