There doesn't seem to be one (or I couldn't find it!). Here are a few guidelines I've picked up from recent patch review comments. Please comment if you have additions and corrections, and maybe we can coalesce this into a page on MDC, with examples and the reasoning behind the guidelines.
- Two space indentation is most common in browser/toolkit code
- Lines need to wrap at 80 characters
- Braces are usually same-line, and only used when surrounding a multiline block
- Use an underscore ("_") as the private prefix for properties and methods
- When wrapping a long expression, the operator goes at the end of the line
- Use spaces instead of tabs
- Adhere to the style of the code you're fixing (or fix it, if it's incorrect!)
- Use inline comments liberally
References: