I've mentioned Acid2 before, but there is actually something to test against now (plus a detailed technical guide that explains how the test works).

From Web Standards Project:

"Today, the Web Standards Project (WaSP) is putting the makers of Web browsers and Web design tools on notice by announcing Acid2, a test designed to expose flaws in the implementation of mature Web standards such as HTML, CSS, and PNG. By making sure their software adheres to the test, the creators of these products can be more confident that their software will display Web pages correctly.

Acid2 has already been found to expose flaws in all tested browsers, including Internet Explorer, Firefox, Opera, and Safari. WaSP hopes that Acid2 will prove useful to browser makers during the development of future versions of their products."

Follow this link to view the reference image, which should be rendered below the text "Have a nice day!" on the test page in the same way that this paragraph is rendered below that text on this page.