css-pseudo-4 defines the terms "tree-abiding pseudo-element" and (a stricter subset) "part-like pseudo-element". Some pseudo elements fall into these categories.

These cause various things to be allowed. In particular, I think the current list is that tree-abiding pseudo-elements are:

• allowed to change the results of getComputedStyle
• allowed after ::slotted()
• support the content property [ Edit: this is what the spec says but it probably doesn't match reality ]

and part-like pseudo-elements do all of the above and also:

• support any CSS properties except those that say otherwise
• can be followed by most pseudo-classes and any pseudo-elements (not yet specified, see [css-pseudo][css-shadow-parts] Define "part-like pseudo-element" concept #10083)

However, there are multiple specifications that define pseudo-elements, some of which define pseudo-elements that probably belong in one of these categories but don't say so, and some of which define pseudo-elements as tree-abiding when they may want the newer part-like definition instead.

We should audit all pseudo-elements and make sure that they're defined appropriately. We should probably also get a group resolution on the result of that audit, since the changes in feature support for many of these pseudo-elements are substantive. (The initial introduction of the part-like term defined a bunch of pseudo-elements as part-like, and I think we could probably use a resolution to approve that.)