merging aria#2410

Hi ARIA editors,

I would very much like to merge 2410 (the stylesheet overhaul / removal).

Since this touches every spec, it easily catches merge conflicts (and
already caught some).

Nothing stops us from continuing these visual design revisions (e.g.,
Scott's comments on AAM table styles), so I see little harm in merging it
now.

Unless anyone objects strongly, I would like to go ahead with the merge.

Best
Peter.