A humble web Programmer, I use the MDC reference pages a lot to make sure that my applications are compatible with the Gecko engine. Of course that means that i will also spot some errors and hopefully will also have some knowledge to give back to the community.
As the CSS_Reference seems to be lacking, I have been adding to it when I get time.
Mozilla_CSS_support_chart - A good chart showing support for various css elements in Firefox/gecko. Currently Orphaned but should be useful for filling in the Browser Compatibility charts in the individual CSS Element articles.