[css-lists] position:marker as layout model for list-style-position:outside list item markers

https://drafts.csswg.org/css-lists-3/#position-marker

An element with position:marker counts as absolutely positioned.

Given the above quote, I have some doubt regarding using position:marker as a description of how layout works for list-style-position:outside list item markers. Making it positioned changes the stacking level in which it is painted. Given how prevalent <li> is on the web, I strongly doubt this would be web-compatible. Non-positioned content that happens to overlap a ::marker must paint on top of it.

FYI, Gecko has implemented ::marker and we made the following tweaks for it to make it compatible with the legacy rendering:

1. when the list item has list-style-position:outside, the display value of the ::marker pseudo-element is blockified
2. the ::marker box inline-size shrink-wraps by default
3. the ::marker box inline-axis margins are not calculated per the CSS2 block-level algorithm (because that would typically make the inline-end margin very large and thus misposition the ::marker)

I think those tweaks are non-controversial since I think they would fall out from making it absolutely positioned anyway, as the spec suggests, so I think it's within the intent of the spec regarding the layout result. But, we do not treat it as absolutely positioned in our box tree.

Read the CSS Lists Level 3 position-marker section, the linked CSS2 stacking-level material, and the CSS Display blockification definition. Compare the proposed model with the listed Gecko ::marker behavior and determine what specification wording is needed to resolve layout and painting compatibility; done means the issue has an agreed, implementable specification outcome.