:root

Supported by Explorer Mac, Mozilla and Safari.

The :root pseudo-element selects the root of all blocks in the page, ie. the Initial Containing Block.

In deference to Explorer 6 Windows this page is in Strict Mode. (It doesn't help, though)

Test stylesheet:

:root {
	background-color: #000000;
	padding: 50px;
}

If the :root selector works you'll see a 50px band of black around this entire page.

:root seems to be the same as html, which, in HTML documents, is logical.