This is great to see! For anyone that doesn’t know what the Acid 2 test is, it is a web standards test that was designed by the Web Standards Project that is intended to pick up rendering flaws in browsers.
It uses HTML and CSS to test certain features and how they are displayed by the browser. Any browser that correctly supports the W3C HTML and CSS 2.0 specifications should render the test correctly. If a browser doesn’t completely/correctly support the features that Acid 2 uses won’t render the page properly.
Interestingly, the test itself doesn’t actually use valid CSS. This is because it was also designed to test how a browser will deal with faulty code.