[css-ruby] Applicability of box properties on base containers and annotation containers

[frivoal]

Currently, the spec says:

The UA is not required to support any of the background properties or outline properties, or any other property that illustrates the bounds of the box on ruby base container boxes or ruby annotation container boxes. The UA may implement these boxes simply as abstractions for inheritance and control over the layout of their contents.

Should we switch that to "must not support"? Seems like whether or not backgrounds apply really ought to be interoperable in order to guarantee the same contrast across implementations.

Neither the margin, padding, border properties nor the any properties that do not apply to inline boxes apply to base containers or annotation containers.

Relatedly, applying backgrounds raises the issue of applying padding: once the edge of the box becomes visually apparent, it becomes important to be able to control its distance from the content. It makes more sense to either apply both or apply neither.

Since we're otherwise treating these two containers are notional rectangles in layout calculations rather than actual boxes, I'd favour making none of these things apply. (Also, no implementation supports M/B/P on these boxes and it's rather nonsensical for inter-character annotation containers.)