[css-fonts-4] Remove duplication of UA requirements

[annevk]

https://drafts.csswg.org/css-fonts/#absolute-size-mapping duplicates requirements from the HTML standard. Perhaps it's okay as a non-normative section, but I think it would be clearer to just strike it and not have the duplication.

[svgeesus]

Got a link to the specific section in HTML? I didn't see it in The h1, h2, h3, h4, h5, and h6 elements and didn't find a section on <font> in the TOC

[annevk]

If you search for xxx-large and h2 { you'll find the requirements. Which goes to show that these are not just duplicative, but wrong in case of h1 through h6 sizing. (At least wrong in that they don't match implementations.)

[svgeesus]

Ok, got rules for parsing a legacy font size which is a subset of what is in CSS Fonts 4 (missing the scaling factor), and Sections and headings

[annevk]

That is a very good point. CSS does actually need to define the values completely, so this is less straightforward than I put it initially.

[svgeesus]

I see the differences in sizes: where does 1.17 come from?

It seems useful to keep the relative sizes for the CSS keywords, but I can see the value in dropping the Hn row and linking to the HTML LS instead.

I also see the conflict with <font> size 1; 'x-small' in HTML, `'xx-small' in CSS.

[annevk]

https://software.hixie.ch/utilities/js/live-dom-viewer/saved/13794 shows computed style. Which appears consistent between browsers and different between font and headings.