Webkit First Browser Engine to Fully Pass Acid3
That same month, another engine, Presto, achieved the 100/100 rendering below, but it still has not achieved "smooth" animation.

The result is as of build r34278. According to the post, recent speedups in JavaScript performance, DOM parsing, and rendering allowed them to pass the smoothness criteria. This means that Safari 4, which is expected to release with Mac OS 10.6, may be the benchmark when it launches.
Acid3 is designed to test Web 2.0 specifications, as defined by WaSP:
The Acid3 Test is designed to test specifications for Web 2.0, and exposes potential flaws in implementations of the public ECMAScript 262 and W3C Document Object Model 2 standards. Collectively known as DOM Scripting, it is these technologies that enable advanced page interactivity and power many advanced web applications such as web-based email and online office applications. As a series of 100 mini-tests, Acid3 has already been found to expose flaws in all tested browsers, including Internet Explorer, Firefox, Opera, and Safari. WaSP hopes that Acid3 will prove useful to browser makers during the development of future versions of their products.