[css-writing-modes] need to define the list of calculations that require a definite inline space #4026

This is a followup to #2239. I was asked to assess how hard it was to implement the change made in #2239 in Gecko, and looking at the spec text there, one piece made it hard for me to understand what the spec requires.

The spec text in question says (both before and after the edit there):

for calculations that require a definite available inline space

but doesn't appear to define which calculations those are. As a potential implementor of this wording, it's not clear to me exactly which calculations those are, and thus how to audit the uses of the available inline space to see which ones need to be corrected.

It would be better if this either:

• cited a definition or term that said which uses of available inline space this fixup applies to, or
• applied this fixup to all uses of available inline space.

(The latter seems simpler, and I'm not sure why the spec doesn't do that.)