The [recently released] Acid3 test, like its predecessor, allows for subjecting modern browsers to advanced testing of its standards compliance. Acid2, which most current browsers pass (including IE8 Beta 1), was mainly focused on CSS rendering.
Acid3 – on the other hand – doesn’t evaluate a single technology. It was designed to test the whole Web 2.0 experience (ECMAScript, W3C Document Model 2, etc.). It basically determines the level of interactivity a browser can deliver.
This extremely hard test evaluates a series of 100 criteria and exposes a number of flaws in all leading browsing suites, including IE, Opera, Safari and Firefox. Upcoming versions of Safari and Firefox are to score in the 80-90 range, whereas IE8 scores in the disappointing sub-50 range.
IE7 has been reported to flunk completely with results not even peaking above the 20% mark.