Define what baseline things align against.

[bfgeek]

See:
https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=6697

Here firefox aligns with the last baseline, and safari aligns with the first.

This needs tests with various combinations of elements. E.g.
https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=6698

[bfgeek]

First baseline will probably be easier on implementations as its more common, and more tested.

[fred-wang]

The core spec now explains the baseline for MathML:
https://mathml-refresh.github.io/mathml-core/#box-model

Algorithms are defined relative to the alphabetical baseline (e.g. they are all aligned in mrow). However, maybe it makes sense to define interpretation of the dominant-baseline property need for vertical alignment has been raised several times in #273 or w3c/mathml#103 for example.

[rwlbuis]

Set the inline offset of the child to inline-offset and its block offset such that the baseline of the child is aligned with the baseline. -> define in terms of first baseline set

[fred-wang]

Set the inline offset of the child to inline-offset and its block offset such that the baseline of the child is aligned with the baseline. -> define in terms of first baseline set

This is for mrow, more generally, we should say that MathML "baseline" means "first baseline" and add tests for all constructions.