The Acid2 test was developed by the Web Standards Project (WaSP) as a way to demonstrate some inconsistencies major browsers have with literal interpretations of the standards. It covers a wide range of HTML and CSS features, including the box model, selectors, objects, strict CSS and comment parsing, CSS display values, generated content, and more. Although it only covers relatively small portions of the standards, it was designed specifically to illustrate some bugs in every major web browser.
Throughout April 2005, Dave Hyatt focused on getting the Safari web browser to pass the Acid2 test, and succeeded by the end of the month, making Safari the first major web browser to render the Acid2 test correctly in its internal developmental builds. By June 2005, Macintosh browser iCab passed the test, followed by the Konqueror browser for Linux. In December, the Prince XML file converter passed the test, and in March 2006, a technical preview of Opera 9 succeeded in passing it.
Now, developmental builds of Firefox join the list of browsers that pass the test. The reflow branch, which has received those last fixes, will eventually merge with the trunk to premier in Firefox 3.0, currently planned for release in 2007. The upcoming Firefox 2.0 will not have any webpage layout engine changes, but will focus solely on user interface improvements instead. The layout engine changes, including the move to the Cairo graphics backend, are very significant and will require more time for testing, so they will all be incorporated in the Firefox 3.0 release.
The only major remaining graphical browser that doesn’t pass the Acid2 test in developmental builds is Microsoft’s Internet Explorer. The developers have said that passing the Acid2 test is not a high priority because their customers are putting demand on other more specific features. After a several-year-long development halt of Trident, Internet Explorer’s layout engine, the developers are currently working to add support for features other browsers have supported for quite a while, rather than focusing on some of the refinement details illustrated by the Acid2 test.