Every programming language, platform, or library has its go-to web site for documentation and API reference: You probably go here if you are a Java developer, here for PHP, here for Ruby, here for jQuery, here or here for your database documentation needs, etc.
For me, it’s often a Google search followed by the process of skimming through dozens of search results trying to find the answer that’s accurate, current, reflects best practices, and works across browsers and operating systems.
Web Platform Docs is a collaborative effort between the W3C, Adobe, Facebook, Google, HP, Microsoft, Mozilla, Nokia, and Opera. The site is still in alpha, but a lot of content has already been contributed by the vendors involved in the project. And as a community, we can all contribute content and expertise to help create the reference site for the web platform.