The Acid2 test is a test of a browser’s ability to properly render CSS and HTML. From the Acid2 Wikipedia page:
The Acid2 test should render correctly on any browser that follows the W3C HTML and CSS 2.0 specifications. Any browser which does not correctly and completely support all of the features which Acid2 uses will not render the page correctly.
Apple’s Safari passed the Acid2 test in October 2005!
Firefox 3 is expected to pass the Acid2 test as well when it is released (beta 2 already does). Internet Explorer 8 is due to ship in the first half of 2008, as is Firefox 3 so it looks as if 2008 will be the year of the standards compliant browser!