Google Doctype is an encyclopedia of the open web. The open web is the web built on open standards: HTML, JavaScript, CSS, and more. The open web is a beautiful soup of barely compatible clients and servers. It comprises billions of pages, millions of users, and thousands of browser-based applications. You can access the open web with open source and proprietary browsers, on open source and proprietary operating systems, on open source and proprietary hardware.
Google has built its business here, on the open web, and we want to help you build here too.
Short answer: because none of the existing resources fit our needs exactly.
For example, many sites offer articles on web development, but few allow continuous improvement once the articles have been written. They are "set in stone," stamped with today's date, discussed briefly, and then forgotten. As the open web evolves, we would like to keep articles section of Google Doctype as up-to-date as possible, for as long as possible. This requires collaborative editing.
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.
The reference section of Google Doctype includes compatibility charts for several browsers, but not all browsers ever made. The limited selection of browsers is deliberate, but we acknowledge that limiting the selection will always be controversial, no matter where we draw the line. We believe that our choices reflect the current state of the web, that is, the browsers that web developers need to care about and test against. The list also reflects the policies that guide product teams inside Google -- these are the browsers that we care about and test against.
We will undoubtedly need to revise our browser selection over time, as new versions come out and old versions become irrelevant. In fact, we have already done so twice during development: once for Firefox 3, and again for IE 8.