Many sites offer browser compatibility charts, but few back up their compatibility claims with repeatable tests. Google believes strongly in the value of test cases and automated regression testing. All of the tests in Google Doctype can be run individually or in groups, using the JSUnit automated testing framework. This allows us to update our charts quickly and easily as new browser versions come out. It also allows you to test in your own browser to see exactly what the compatibility claims mean.
Many sites offer developer resources for the open web, but few sites make those resources available under open licenses. You can read, but you can’t download. Or you can download, but you can’t redistribute. Google makes it easy to download the entire encyclopedia and take it with you, publish it on your private intranet, or package it for redistribution.
Source: Google Doctype