[css-inline] baseline values for dominant-baseline and alignment-baseline

[kojiishi]

Some questions and suggestions to the values of the 'dominant-baseline' and the 'alignment-baseline':

1. I believe 'alphabetic' means the baseline of Latin scripts. Is this correct?
2. If so, could we use 'latin' or 'roman', since every script has alphabets, unless this is already implemented and used? I don't know the differences between Latin and Roman, but OpenType uses 'romn' tag in the baseline table.
3. What are the difference between 'ideographic' and 'text-bottom', when both says the bottom? Does one include internal leading while the other doesn't? Or is the diff only appear in inline replaced elements?
4. Could we not use the term 'ideographic' for the under-side baseline? Opposite from the 'alphabetic` case, I guess this is from OpenType uses 'ideo' for the bottom in its baseline table, but this value is not for ideographic scripts, and thus is confusing for authors of ideographic scripts.

FYI, @eroux is working on BASE table support in HarfBuzz.

[fantasai]

@kojiishi I think the problem here is that we have existing compat restrictions due to SVG.

It's an open question what exactly text-bottom refers to, but ideographic refers to the ideo baseline, whereas I think text-bottom refers to the descent metric. They might be equivalent in many fonts, but it might not be true in ones that cover a wide range of writing systems.

Lmk if you think there's anything else to solve here, or if we should close the issue. :)