[css-writing-modes] Application of text-combine-upright should be defined in terms of typographic mode

[nt1m]

Gecko seems to check the direct writing mode values: https://searchfox.org/mozilla-central/rev/778df10b835ea87def2bfdbb83810ee36bc22c94/servo/components/style/style_adjuster.rs#338-341

but it seems like checking the typographic mode would make sense.

This is not really specified in the spec.

[fantasai]

Gecko is checking the typographic mode by looking up the writing-mode value. (That test doesn't trigger on the sideways-* values.) Which seems correct: the point of the sideways-* values was to use horizontal typesetting conventions, graphically rotated sideways.

Agreed the spec should clarify this, and we should have a test for it.

[nt1m]

Testcases already exist, I think this is mostly an editorial change.

css/css-writing-modes/text-combine-upright-sideways-001.html
css/css-writing-modes/text-combine-upright-sideways-002.html