This diff also showed up on enwikivoyage pages.

I've added this to the known issues page.

So, this bug seems specific to Vector skin and not Parsoid HTML or any transformations done in core.

These classes are being placed on a div wrapping the h1 heading, but it doesn't seem like DT wraps the h1 similarly. At least there's no mw-heading1 in the legacy output.

$div->setAttribute(
	'class', 'mw-heading mw-heading' . $section->hLevel
);

https://github.com/wikimedia/mediawiki/blob/master/includes/OutputTransform/Stages/HandleParsoidSectionLinks.php#L108

As I understand @matmarex's https://www.mediawiki.org/wiki/Heading_HTML_changes there /will/ be a mw-heading1 added once the legacy parser adopts the new heading HTML, but there's not one from the legacy parser currently (and that's expected). Any CSS rule which targets <h1> should also target .mw-heading1 in the way shown by https://www.mediawiki.org/wiki/Heading_HTML_changes#Skins_and_sitewide_styles.

With

.mw-heading2, .mw-body h2 {
    font-size: 1.5em;
}

the nested heading has its font-size scaled by its parent though, so it's applied twice,

<div class="mw-heading mw-heading2">
  <h2 id=...>

DT works around this by setting,

.ext-discussiontools-init-section.mw-heading2 {
	// stylelint-disable-next-line declaration-no-important
	font-size: inherit !important;

https://github.com/wikimedia/mediawiki-extensions-DiscussionTools/commit/8271401e73dd81d7205f947b19c301b2edc0d83f

In T359881#9689173, @ABreault-WMF wrote:

DT works around this by setting,

.ext-discussiontools-init-section.mw-heading2 {
	// stylelint-disable-next-line declaration-no-important
	font-size: inherit !important;

https://github.com/wikimedia/mediawiki-extensions-DiscussionTools/commit/8271401e73dd81d7205f947b19c301b2edc0d83f

Whoops, no, it looks like it was fixed in T357929 by,
https://github.com/wikimedia/mediawiki-skins-Vector/commit/9f1b94ce20a60112b65dc25baf9014e46133ae64

That's just missing .mw-heading1 h1

Change #1017105 had a related patch set uploaded (by Arlolra; author: Arlolra):

[mediawiki/skins/Vector@master] Add h1 headings to fix for applying font-sizes

https://gerrit.wikimedia.org/r/1017105

Change #1017105 merged by jenkins-bot:

[mediawiki/skins/Vector@master] Add h1 headings to fix for applying font-sizes

https://gerrit.wikimedia.org/r/1017105

On enwiki, which is still on 1.42.0-wmf.25, we see the issue,
https://en.wikipedia.org/wiki/Special:Version
https://en.wikipedia.org/wiki/User:ABreault_(WMF)/sandbox?useparsoid=0
https://en.wikipedia.org/wiki/User:ABreault_(WMF)/sandbox?useparsoid=1

But on mediawikiwiki, which is now has 1.42.0-wmf.26 deployed, it is resolved.
https://www.mediawiki.org/wiki/Special:Version
https://www.mediawiki.org/wiki/User:ABreault_(WMF)/sandbox?useparsoid=0
https://www.mediawiki.org/wiki/User:ABreault_(WMF)/sandbox?useparsoid=1

It should be safe to close this out.

Amazing, thank you so much @ABreault-WMF!

Test Result - Prod

Status: ✅ PASS
Environment: enwiki
OS: macOS Sonoma
Browser: Chrome
Device: MBA
Emulated Device:NA

Test Artifact(s):

Test Steps

See Parsoid rendering of https://en.wikipedia.org/wiki/Talk:WrestleMania_38?useparsoid=1
See non-Parsoid rendering of that page with https://en.wikipedia.org/wiki/Talk:WrestleMania_38?useparsoid=0
Expected results
✅ AC1: The h1- headings would be rendered identically in both Parsoid and non-Parsoid renderings

parsoid=0	parsoid=1