2.1.20 [CSS-Level2-2009] Section 5.11.3, The dynamic pseudo-classes: :hover, :active, and :focus

V0194:

The specification states:

 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.