The dominant-baseline property has the following values:

auto | text-bottom | alphabetic | ideographic | middle | central | mathematical | hanging | text-top 

And the alignment-baseline property has the following values:

baseline | text-bottom | alphabetic | ideographic | middle | central | mathematical | text-top 

Besides auto and baseline, they're identical except alignment-baseline is missing hanging. I assume that it's meant to have it, especially since it was an accepted value in SVG 1.1 and it is not listed as having been removed in SVG 2.

I can submit a small PR to put the hanging value into alignment-baseline if this is the case.