The Acid Test 2
Acid2 is a test created by the WaSP (Web Standards Project) that can be used to show if a browser implementation of CSS and XHTML adheres to standard. In fact the tricky part here is CSS.
The Acid2 test is expected to render correctly on any browser that follows the W3C HTML and CSS specifications. Of course a browser that does not correctly support all of the features used in Acid2, will not render the page correctly.
This is simply the first browser to render correctly the Acid2 test (Safari updated for MacOS X 10.4.3). It's a bit surprising that while the test was published in April 2005 and Safari passed it on October 2005 [ Konqueror passed it a month later ], the "considered-most-standard" Mozilla does not. Not yet.
In fact the private builds of Dave Hyatt supported Acid2 since 14 days after its publication.
Gecko: Mozilla, Firefox
We can just say that this browser family appears not to support Acid2 and it's not clear if they are going to support it.
Opera 9 is the first windows browser to pass the test (10 March 2006). However we are talking about a prerelease, not a "stable" version.