I'm already aware of some browser-specific DOM documentation out there (Gecko DOM Reference for Firefox, MSDN for Internet Explorer and, of course, the ECMA and W3C specs) but I wonder if someone has compiled a good API reference that covers all common browsers (Firefox, Internet Explorer, Opera, Safari, Chrome...).
The compatibility tables by quirksmode.org are a good resource but they don't actually document stuff (it's not their purpose).
P.S. A downloadable format (such as CHM) would be a nice bonus ;-)
This was one of the best sites but has not been updated since at least July. 2009, iirc.
You'll find a lot of useful info here, too.