This is a pseudo-class indicating a virtual piece of a document -
the rendering viewport. As such, its use is limited and should not be
combined with other elements in a selector.
Example
Ext/Doc: :viewport
{ background-color: yellow }
(turns the background of the viewport yellow.)
In-Line: NA
Browser Peculiarities
Netscape 6 doesn't seem to apply border, margin, padding or other
properties to this pseudo-class. The only obvious properties that had
an effect (in my admittedly limited tests) were color and background.
Be sure to test thoroughly if you intend to use this pseudo-class.