[css-contain] Describe generalized goals containment #10382
Description
The way style containment is specified is unlike how layout/size/paint containment is specified.
Layout/size/paint describe what kind of high level effects these containment modes have. They include some specifics about how that affects certain properties when that's necessary, but they still give a general sense of what they do, so that when you add some new property, you can consider whether it needs to behave in any particular way in the face of containment to respect those high level goals. They might not be perfect in terms of how they do that, but at least they try.
Style containment only describes itself in terms of the effects it has on some specific properties, without giving any generalized objective. This makes it harder to reason about what style containment should do (if anything) to newly added properties (anchor names, scroll timelines…)
I also wonder if we should add an entry to propdef tables about containment, to remind anyone who creates a new property that they need to consider whether containment changes any part of the behavior of that property.