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."