[css-2026] Proposed new stability categorization

[svgeesus]

New categorization (adding category B):

• A) REC-level stability -- might be REC, might be CR without an implementation report, or a few bugs in implementations etc.

• B) reliable CR-level stability -- should be in CR, or close and has usable shipping implementations

• C) stable spec with limited implementation experience (could be destabilized once we try to implement)

• D) unreliable spec with shipping, mostly usable implementations

Originally posted by @fantasai in #9770

[svgeesus]

Posting this in a new issue since the proposal was kind of buried inside an unrelated issue.