Acid2
Acid2 is a test case designed to "push the limits of HTML, Cascading Style Sheets (CSS), and PNG support in browsers and authoring tools". It was created by the Web Standards Project, and follows an earlier test from 1997 known as Acid.
The page is designed to quickly identify web page rendering flaws. If rendered properly, the page will show a smiley face. At the time that the test was released, no browsers rendered the page properly.
On April 27, 2005, Dave Hyatt announced that a specially patched version of Safari (with WebCore as its layout engine, a spin-off from KDE's KHTML) had passed the test. On June 4, 2005, Allan Sandfeld Jensen announced that the KDE web browser Konqueror—for which KHTML was originally created—had been updated to pass the test as well, partially benefitting from the Safari patches.
The programmers of Mozilla and Opera have also been putting effort into fixing the bugs identified by the test.
Microsoft has been working on improving CSS support, but does not plan on passing this test for the upcoming release of Internet Explorer 7 [1].