CSS priority of mapped attributes

[fred-wang]

Note that this issue excludes rules that are already in the UA stylesheet, since specificity is defined by CSS. This is about attributes that are said to be "mapped to" a CSS property.

This was discussed in #171 ; for example, which color is used for <math style="color: blue" mathcolor="red">? The same question arises in HTML / SVG for mapped attributes. We should probably use the same wording / definitions as in these specifications. I don't believe there is a compat issue, since AFAIK all browsers behave the same for mapped attributes.

For example the color attribute in HTML: https://html.spec.whatwg.org/multipage/rendering.html#phrasing-content-3:font

[fred-wang]

Proposal: they presentational hints == author-level zero-specificity presentational hints part of the CSS cascade

https://www.w3.org/TR/css-cascade-3/#cascading
https://html.spec.whatwg.org/multipage/rendering.html#presentational-hints

[fred-wang]

So reading again https://www.w3.org/TR/css-cascade-3/#cascading:

"For the purpose of this step, declarations that do not belong to a style rule (such as the contents of a style attribute) are considered to have a specificity higher than any selector."

while https://html.spec.whatwg.org/multipage/rendering.html#presentational-hints

"author-level zero-specificity presentational hints"

so style attribute wins over mapped attribute. Of course https://www.w3.org/TR/css-cascade-3/#cascading gives details for other rules.