Google Doctype is open to contributions from anyone. Technically speaking, it is a wiki, similar to Wikipedia or the individual project wikis on Google Project Hosting. On the one hand, most pages on Wikipedia are editable by anyone, even without logging into an account. On the other hand, Google Project Hosting wikis are only editable by members of that particular project. Google Doctype is somewhere in between: pages are editable by anyone who is logged into Google services with a Google account. If you do not have a Google account, you can create a new account for free.
To edit a page in Google Doctype, you need to be looking at it. Then simply click the "Edit this page" link at the top of the page. To create a new page, just click the "New page" link from any page. Please follow the naming conventions as best you can.
New facts, especially in the reference sections on HTML, JavaScript DOM, and CSS. Facts about how a supported browser's behavior deviates from other browsers is always useful, no matter how trivial the difference may seem.
Corrections to existing facts. Has a statement become obsolete due to new browser versions, new standards, or previously undiscovered phases of the moon? Please correct it. Don't ask, just do it.
New HOWTO articles about accomplishing a specific goal across all supported browsers. Articles backed by sample code are even better. All sample code must be licensed under the BSD license and must work in all supported browsers.
Links to useful and related content elsewhere. Google Doctype is not a closed system, and there will always been people who wish to publish content elsewhere (on their own blogs, in newsgroups, or even other web development sites where they can get paid for it). Virtually every page in Google Doctype contains a "Further reading" section with links to relevant background material and related articles elsewhere on the web. If the page you're looking at doesn't have such a section, please add it. Don't ask, just do it.
Content unrelated to web development. Google Doctype is for web developers, by web developers. It is not a general encyclopedia like Wikipedia.
Content about proprietary technologies such as Adobe Flash, Microsoft Silverlight, Apple QuickTime, or ActiveX. These proprietary plug-ins, while widely used, are not the focus of this project.
The only exception to this rule is security-related articles. Even if a particular web application does not employ proprietary plug-ins, the presence of such plug-ins in the user's browser environment may pose security risks that developers need to be aware of. Google Doctype includes an entire section on security issues in Adobe Flash. Similar articles are always welcome.
Pure opinion pieces, without a HOWTO component. Having opinions is OK; having nothing but opinions is not. An article that explores the question "what's the best way to mark up the name of a ship?" is OK. An article that explores the question "what's the most standards-compliant browser on Microsoft Windows?" is not. Different people will choose different browsers; Google Doctype is not the place to convince them to change, and web developers will need to accomodate all supported browsers in any case.
If you have nothing but opinions and feel the need to express them on a regular basis, you should start a blog.
Spam, trolls, vandalism, random acts of unkindness. Vandalism will be reverted; trolls will be ignored. Spammers will be banned from Google Doctype and may have their Gmail accounts revoked.