Style Sheet Property Reference - Pseudo-Classes
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The a element has readily distinguishable states: a link that has not been visited, a link being clicked on, a link that has been visited in recent history. These states are called pseudo-classes; they work like class selector definitions but don’t have to be labeled as such in their element tags. A pseudo-class always operates as a kind of modifier to another selector. In the following example, notice how the :hover pseudo-class operates on all a elements in one rule, and applies an extra color property to an a element singled out by its ID:
a {text-decoration: none }
a:hover {text-decoration: underline}
#specialA:hover {color: red}
The classness of a pseudo-class is not always based on an element’s state. Document tree context, page position (right or left), and even language are examples of the pos sibilities that pseudo-classes afford. For example, the :first-child pseudo-class turns its associated element into a special class (i.e., a class capable of defining its own style propertys) whenever the element is a first child element in a document tree. Thus, the following style rule applies a different font size for every p element that is the first child of any container with the class name section :
.section > p:first-child {font-size: 110%}
The use here of the > child selector limits the scope of the p:first-child pseudo-class to first children of specific containers. Removing the child selector would cause the rule to apply to any p element that is the first child of any other container.
Table 4-3 provides a summary of pseudo-classes supported by CSS2. Implementation in mainstream browsers is sporadic.
Table 4-3. CSS2 Pseudo-classes
| Name | IE/Windows | IE/Mac | Moz/Saf/Op | CSS | Description |
|---|
| :active | 4 | 4 | all | 1 | An a element being clicked on by the user |
| :first | n/a | n/a | n/a | 2 | First page of a document (with @page declaration) |
| :first-child | n/a | 5 | all | 2 | Any element that is the first child of another element |
| :focus | n/a | 5 | all | 2 | Any element that has focus |
| :hover | 4 | 4 | all | 2 | An element that has a cursor on top of it (only a elements in IE 4-6 and in IE 7 quirks mode) |
| :lang(code) | n/a | 5 | n/a | 2 | An element with the same language code |
| :left | n/a | n/a | n/a | 2 | A left-facing page (with @page declaration) |
| :link | 4 | 4 | all | 1 | An a element that has not yet been visited |
| :right | n/a | n/a | n/a | 2 | A right-facing page (with @page declaration) |
| :visited | 4 | 4 | all | 1 | An a element that has been visited within the browser’s history |
CSS3 introduces a large collection of new pseudo-classes, some of which are already implemented in the latest mainstream browsers. The largest group of new selectors allow you to assign style properties to elements that meet very specific contextual criteria, such as every other row of a table, without burdening the HTML markup with lots of class attributes. Some selectors allow style sheets to complement or replace the browser’s default rendering for states, such as a disabled element or a “checked” button. Table 4-4 lists CSS3 selectors as of the latest W3C Working Draft available before going to press.
Table 4-4. CSS3 Pseudo-classes
| Name | IE | Mozilla | Safari | Opera | CSS | Description |
|---|
| :checked | n/a | n/a | n/a | 9 | 3 | A radio or checkbox button is checked |
| :disabled | n/a | n/a | n/a | 9 | 3 | A focusable element is disabled |
| :empty | n/a | 1.0.1 | all | n/a | 3 | An element containing no child nodes |
| :enabled | n/a | n/a | n/a | 9 | 3 | A focusable element is enabled |
| :first-of-type | n/a | n/a | n/a | n/a | 3 | Any element that matches the tag name and is the first child of a parent element |
| :invalid | n/a | n/a | n/a | 9 | n/a | A Web Forms 2.0 control element whose value is invalid |
| :last-child | n/a | 1.0.1 | n/a | n/a | 3 | Any element that is the last child of a parent element |
| :last-of-type | n/a | n/a | n/a | n/a | 3 | Any element that matches the tag name and is the last child of a parent element |
| :not(selector) | n/a | n/a | n/a | n/a | 3 | Elements not matching selector |
| :nth-child(an+b) | n/a | n/a | n/a | n/a | 3 | Every b th child element in groups of a |
| :nth-last-child(an+b) | n/a | n/a | n/a | n/a | 3 | Every b th child element counting backwards from the last sibling, in groups of a |
| :nth-last-of-type(an+b) | n/a | n/a | n/a | n/a | 3 | Every b th child element with the same tag name counting backward from the last sibling, in groups of a |
| :nth-of-type(an+b) | n/a | n/a | n/a | n/a | 3 | Every b th child element with the same tag name in groups of a |
| :only-child | n/a | n/a | n/a | n/a | 3 | An element that has no siblings |
| :only-of-type | n/a | n/a | n/a | n/a | 3 | An element that has no siblings with the same tag name |
| :root | n/a | n/a | n/a | n/a | 3 | The HTML element |
:target | n/a | n/a | n/a | n/a | 3 | An element contained by an anchor |
Please check back next week for the continuation of this article.
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 This article is excerpted from chapter four of Dynamic HTML: The Definitive Reference, Third Edition, written by Danny Goodman (O'Reilly; ISBN: 0596527403). Check it out today at your favorite bookstore. Buy this book now.
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