[css-ruby-1] What properties apply to annotation containers?

[fantasai]

We don't very clearly specify which properties do or do not apply to annotation containers and base containers. There's a specific issue on box-related properties, but we need a more generic definition as well.

Note: Annotations containers are defined to exactly contain the margin boxes of all their child annotations, except for inter-character annotations which are zero sized per #5994 because they interleave with the bases. They're currently defined to be mostly unstyled, so that layout engines are not required to implement them as fully-functional boxes.

Proposal: Create allow list of properties that apply to annotation containers, say all other properties don't apply.

Here's several categories of properties to consider:

Definitely should apply:

• Anything explicitly said to apply to annotation containers
• visibility
• display
• direction
• unicode-bidi
• text decoration

Probably should apply, mostly about style computation:

• transitions
• animations
• counters
• quotes

Maybe apply? (Doesn't require the box to draw, but might be hard to implement without a real box.)

• opacity?
• position + inset (?)
• transforms (?)
• scroll-snap (? seems unnecessary, but not a problem if we know the geometry)

Probably not, because requires the box to draw, so it can't be implemented as an abstract concept anymore (but if we say yes, probably yes to all of them):

• background
• clip-path
• masking
• filter effects
• mix-blend-mode
• outline
• margins/border/padding (already resolved not to apply, but probably should if we want backgrounds)

See also #5999

Note: Anything that inherits will inherit through to the annotation boxes and apply there. Inheritable properties not listed above don't apply.