The Acid tests are designed to check how compliant a browser is with the web standards laid down by the Web Standards Project (WaSP). Acid 3, reckoned to be the most stringent test, was introduced earlier this year and WebKit is first of the industry's browsers to yield a good result under the test.
The achievement puts WebKit ahead of other browsers in the race to meet the Acid 3 test - although other leading browser builders have rejected the test as irrelevant. Chris Wilson, platform architect for Microsoft's Internet Explorer, has stated that while Acid 3 is "a collection of interesting tests," it is not Microsoft's goal to comply.
And don't all you open-source lot start tutting and muttering "typical Microsoft" either. Mozilla's Firefox 3 team has also dismissed Acid 3 saying: "None of the issues on the Acid 3 list are important enough." ®