[css2] Apply the patch from css-ui/box-sizing

[frivoal]

CSS2, and in particular the Visual formatting model details makes a non rigorous use of terms like width, min-width, max-width, height, etc, either to refer to the value of these properties, or to the inner sizes. That was not particularly problematic when initially written, but has become more of a problem since the introduction of the box-sizing property.

CSS-UI-3 has a disambiguating patch in its section defining box-sizing, but that is hard to read, and as per the resolution in #2458 (comment), it should be applied and inlined int CSS2.

[gsnedders]

This is just s/width/content width/ and s/height/content height/, right?