Fixes #21740 (closed).
This removes a dedicated Normalize.css from the project, merging the relevant parts into Reboot. With the upcoming removal of "opinionated" styles that we rely on, combined with the growing overrides and inconsistencies between Normalize and Reboot, this should simplify things a bit.
This is missing a good amount of attribution (most obviously from Normalize), organization of rulesets, and lacking any real testing yet.
Looking at other open issues and PRs, this also addresses a few bugs.
Closes #21657, fixes #21588 (closed), fixes #21652 (closed).