Support for CSS alignment properties

[fred-wang]

This was discussed elsewhere but I'm opening this for the record...

See https://www.w3.org/TR/css-align-3/#intro

It seems many of these properties could be used as a replacement for legacy attributes like those from w3c/mathml#30 ; or for people willing to perform their own alignment in math.

[fred-wang]

consensu from 2020/06/29: move to level-2

[fred-wang]

@davidcarlisle can you please move this and other issues with MathML Core label to https://github.com/w3c/mathml/labels/MathML%20Core ? Thanks

[fred-wang]

I removed MathML 4 label as I don't think the full spec talks about interaction with CSS or layout. This kind of thing is done in core.