Much better documentation is available at sitepoint's web site:
http://reference.sitepoint.com/html
and
http://reference.sitepoint.com/css
(See screenshots to compare docs for "background-color".)

Would it be possible to either

• generally use the sitepoint docs instead?
• make options for W3C and sitepoint?
• make a general option to configure the documentation provider?
  For example using templates to specify the URL given the
  tag and attribute names, resp. css-property. To make this really useful
  one would have to be able to specify a javascript snippet that would run
  prior to rendering the page (to remove unwanted content).

I guess at least for the first two options, you would have to check with sitepoint concerning the legality.