I think the Acid test site explains the process, it's basically a test of how well the bowser uses the current web standards. Those are changing as html5 modules and CSS3 become better understood. Of course, html5 is still a work in progress so the test is often updated. There are some parts that different browsers use with browser-specific property labels. As the become accepted by the standards group, the browser makers may have to change the way they work because people don't want to keep duplicating bits of code just for one or two browsers...they had enough of that with Microsoft!!
