I think you're confusing "most marketing checkboxes checked" and "best support" in terms of Webkit and HTML5 (and also in terms of Webkit and "finest rendering engine").
For example, Webkit is the rendering engine with the worst CSS 2.1 support of the big 4, last I checked, based on the results of the CSS 2.1 test suite. Yes, it claims to implement all of it, but the quality of the implementation is not as good as the others.
For another example, Webkit is the only rendering engine I know that claims to implement CSS3 Selectors but purposefully does so incorrectly because they think doing it right would be "too slow".
This is one thing IE8/9 have actually been doing right. When they commit to implementing a feature they've been implementing it well. The feature set is smaller than webkit, but there are fewer gotchas in the features they do support...
For example, Webkit is the rendering engine with the worst CSS 2.1 support of the big 4, last I checked, based on the results of the CSS 2.1 test suite. Yes, it claims to implement all of it, but the quality of the implementation is not as good as the others.
For another example, Webkit is the only rendering engine I know that claims to implement CSS3 Selectors but purposefully does so incorrectly because they think doing it right would be "too slow".
This is one thing IE8/9 have actually been doing right. When they commit to implementing a feature they've been implementing it well. The feature set is smaller than webkit, but there are fewer gotchas in the features they do support...