2.1.20 [CSS-Level2-2009] Section 5.11.3, The dynamic pseudo-classes: :hover, :active, and :focus
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Interactive user agents sometimes change the rendering in response to user actions. CSS provides three pseudo-classes for common cases: The :hover pseudo-class applies while the user designates an element (with some pointing device), but does not activate it. For example, a visual user agent could apply this pseudo-class when the cursor (mouse pointer) hovers over a box generated by the element. User agents not supporting interactive media do not have to support this pseudo-class. Some conforming user agents supporting interactive media may not be able to support this pseudo-class (e.g., a pen device). The :active pseudo-class applies while an element is being activated by the user. For example, between the times the user presses the mouse button and releases it. The :focus pseudo-class applies while an element has the focus (accepts keyboard events or other forms of text input).
Quirks Mode and IE7 Mode (All Versions)
The :focus and :active pseudo-classes are not supported for any elements other than <a>.
IE8 Mode (All Versions)
The :focus pseudo-class is not supported when followed by a combinator.
All Document Modes (All Versions)
The :hover pseudo-class applies to an element only when the mouse pointer is hovering over the content of that element’s box.