Remove UA style for h1-h6 in section (et. al.) and hgroup

[zcorpan]

If/when #7829 is merged, the following UA style doesn't make sense. Can it be removed?

In the following CSS block, x is shorthand for the following selector: :is(article, aside, nav, section)

@namespace url(http://www.w3.org/1999/xhtml);

x h1 { margin-block-start: 0.83em; margin-block-end: 0.83em; font-size: 1.50em; }
x x h1 { margin-block-start: 1.00em; margin-block-end: 1.00em; font-size: 1.17em; }
x x x h1 { margin-block-start: 1.33em; margin-block-end: 1.33em; font-size: 1.00em; }
x x x x h1 { margin-block-start: 1.67em; margin-block-end: 1.67em; font-size: 0.83em; }
x x x x x h1 { margin-block-start: 2.33em; margin-block-end: 2.33em; font-size: 0.67em; }

x hgroup > h1 ~ h2 { margin-block-start: 1.00em; margin-block-end: 1.00em; font-size: 1.17em; }
x x hgroup > h1 ~ h2 { margin-block-start: 1.33em; margin-block-end: 1.33em; font-size: 1.00em; }
x x x hgroup > h1 ~ h2 { margin-block-start: 1.67em; margin-block-end: 1.67em; font-size: 0.83em; }
x x x x hgroup > h1 ~ h2 { margin-block-start: 2.33em; margin-block-end: 2.33em; font-size: 0.67em; }

x hgroup > h1 ~ h3 { margin-block-start: 1.33em; margin-block-end: 1.33em; font-size: 1.00em; }
x x hgroup > h1 ~ h3 { margin-block-start: 1.67em; margin-block-end: 1.67em; font-size: 0.83em; }
x x x hgroup > h1 ~ h3 { margin-block-start: 2.33em; margin-block-end: 2.33em; font-size: 0.67em; }

x hgroup > h1 ~ h4 { margin-block-start: 1.67em; margin-block-end: 1.67em; font-size: 0.83em; }
x x hgroup > h1 ~ h4 { margin-block-start: 2.33em; margin-block-end: 2.33em; font-size: 0.67em; }

x hgroup > h1 ~ h5 { margin-block-start: 2.33em; margin-block-end: 2.33em; font-size: 0.67em; }> 

https://html.spec.whatwg.org/multipage/rendering.html#sections-and-headings

[domenic]

Seems extremely unlikely.

[annevk]

Unlikely, but also quite unfortunate. I'd certainly support someone investigating this.

cc @whatwg/css

[zcorpan]

The hgroup special styling could probably be removed since hgroup usage is a lot lower than the sectioning elements, and multiple headings in hgroup is non-conforming with #7829. And it's not implemented in any of Gecko, Chromium, WebKit!

Demo:
https://h1-ua-style.glitch.me/

Maybe it's possible to remove the other rules also; author CSS to set the font-size was needed to get consistent style in older browsers (in particular IE8). But this would need some research.

[zcorpan]

h2-h5 in hgroup styling removed in #7886

[zcorpan]

Adding agenda+ to check interest in adding a use counter for h1 with a sectioning element ancestor and no Author Origin style for font-size.

[zcorpan]

We didn't get to this issue in the triage meeting today. Optimistically tagging @mfreed7 :)