For those using Mac OSX and who have just updated to Mac OSX 10.4.3, Safari now passes the "Acid Test".

The Acid2 test was concieved by the Web Standards Project (WaSP) to promote World Wide Web Consortium Web-based content standards.

So what is the Acid Test? This is best explained from the pages of WaSP itself:
http://www.webstandards.org/act/acid2/guide.html

These features are part of existing standards but haven't been supported by major browsers. Acid2 tries to change this by challenging browsers to render Acid2 correctly before shipping.

Take your browser for a test here:
http://webstandards.org/act/acid2/test.html