In an unexpected (but not undeserved) moment of Nielsenism, W3C has published a list of things which browsers should do, but often don't. These aren't so much focused on HTML non-compliance, but on the basic behaviour of browsers such as "Don't treat temporary HTTP redirects as permanent ones"

The issues which W3C has largely boil down to "Hey! We're not sitting down and producing robust standards for a laugh, y'know"