Should this go to https://quirks.spec.whatwg.org/ instead of HTML?

@Loirooriol no, HTML already has quirks mode style rules like this. Eventually, all quirks should be upstreamed to the appropriate spec instead of being defined in a delta spec.

Marking "do not merge yet" for #10958 (comment) (also see earlier comments).

I've updated this PR to also apply this behavior in standards mode and to include menu and dir in the selector, per web compat research findings. I've also updated the tests.

Slight preference to align on current WebKit / Firefox Nightly behavior (just quirks and no dir / menu) fwiw... There may be other compat impact of making the change to computed styles in standards mode so if we can get away with it seems preferable, specially if the visual impact is minimal

(Plus those selectors are not particularly cheap)

I've changed this again to only apply in quirks mode.

I doubt we really need menu, dir in the style fwiw, the websites that looked better in Firefox also looked better in Safari builds with WebKit/WebKit#37176 applied (which does not have menu, dir in its UA styles). So I suspect this is more due to Chrome/old WebKit considering li inside menu as standalone li and applying quirky behavior there, rather than due to Firefox UA styles covering menu, dir.

I could live either way on including menu, dir though, but I'd love to understand the reasoning behind this better.

For example http://old.lawjusticediv.gov.bd/ with list-style-position: inside on menu > li renders the markers on their own lines, because each list item contains a div.

In WebKit the marker is on the same line anyway, but that seems like a bug, if I understand the spec correctly.

No special effect. (The ::marker is an inline element at the start of the list item’s contents.)

https://drafts.csswg.org/css-lists/#valdef-list-style-position-inside

Demo: https://software.hixie.ch/utilities/js/live-dom-viewer/saved/13478

Apart from making a few pages render better, I think it also makes more sense to make dir and menu render like ul, since the spec includes other UA style rules for them (e.g. list-style-type and padding-inline-start). menu is a conforming element. dir is not, but if we want to get rid of that from the UA stylesheet, it should be removed everywhere.

https://software.hixie.ch/utilities/js/live-dom-viewer/saved/13479

I see now that the above page is in standards mode, but I intentionally analyzed more standards mode pages than quirks mode pages (to understand impact of quirks-mode-only vs all modes).

Yes, WebKit doesn't treat inside markers as normal inline boxes. See relevant discussion: w3c/csswg-drafts#5276 (comment)