The Acid2 Test
Way back when, there was an Acid test for testing CSS box model implementations in browsers. This simple test was used by browser developers to ensure proper alignment with appropriate standards.
Now, the Web Standards Project has introduced the second generation, the Acid2 Test, to further help browser makers tackle proper CSS, PNG, and HTML implementations, among other things. It really is an ingenius little page, make sure you read the description of how the page is constructed.
Disclaimer: Any viewpoints and opinions expressed in this article are those of Nicholas C. Zakas and do not, in any way, reflect those of my employer, my colleagues, Wrox Publishing, O'Reilly Publishing, or anyone else. I speak only for myself, not for them.