[css-ruby] Limiting leading/trailing space with ruby-align:space-around

[frivoal]

The behavior currently prescribed by ruby-align:space-around is not quite the one that JLREQ or Simple Ruby calls for: They limit the amount of space at the edges to 1em of the ruby annotation font size, to ensure that the annotation obviously spans across the entire annotated base text. Should we encode this limit into the definition of space-around?

Simple Ruby: https://www.w3.org/TR/simple-ruby/#group-3
JLREQ: https://www.w3.org/TR/jlreq/#fig2_3_21

See also Antenna House's user manual, which calls for the same behavior. Not suggesting we add support for the property they have to control this in L1, but its initial value matches.

[himorin]

The route reason of this is how to find boundaries of ruby base from attached annotation characters as in JLReq note. If we don't have a limit of maximum 1em (or 1.5em) of annotation character (or half of base character) for combination of long base and short annotation, we could easily miss the outermost characters of base to be not in the ruby base text.
For cases shown in examples in simple-ruby or JLReq, it is quite easy to tell where is the ruby base text in a long text like:

but, in some cases like in animations (and daily use cases in Japanese cultural area) it's quite common to place short annotation for long title, like あの日見た花の名前を僕達はまだ知らない。 (title contains '。', and officially annotated as 'あの花' or 'あのはな') or 私、能力は平均値でって言ったよね！ (title contains '！', and annotated as 'のうきん'), and it's quite difficult to tell their boundary if someone does not have enough information, like:

[frivoal]

I suspect there's probably a need for some flexibility down the line to relax this boundary, and that we could, in level 2, introduce some property, as antenna house does, to change how big the limit is or remove it, but I think the default behavior should include that limit.