This is a pseudo-class indicating a virtual component of a document -
the rendering canvas. Because it doesn't exist in the document tree, its
use is limited and should not be combined with other elements in a selector.
Example
Ext/Doc: :canvas
{ border: 30px solid red }
(make a 30 pixel, solid, red border on the canvas, underneath
other rendered element content.
In-Line: NA
Browser Peculiarities
In Netscape 6, this pseudo-class will behave the same as BODY or :root
or :viewport as a selector will. But if you use a property like 'border',
you will see the important difference. The rendering box for :canvas is
not AROUND any document elements...it is rendered UNDERNEATH
the document, much the same way that the 'background-image' is. The
biggest difference between "body { background-image: url() }" and
":canvas" is that the full range of CSS properties can be rendered
"underneath" the canvas in the latter case.