@tabatkins I agree with that generally as an objective, that is:
Normatively state in CSS 2.1 that the grammar is defined by CSS 3 Syntax, with the key exception that we explicitly exclude any requirement to support scientific notation to maintain the fact that we are not adding any new features to CSS 2.x.
We should note any such exception like that in such a way to not discourage implementations from implementing all of CSS 3 Syntax, that is, something like:
For the purposes of this inclusion by reference, the scientific notation feature is excluded from CSS 2.1 in order to not add any new features to CSS 2.1. Implementations may (and are encouraged to) implement the full CSS 3 Syntax specification, and we expect any new or modern implementations to do so.
All this being said, assuming we can agree on this path forward for resolving this issue, I would like this issue to not be a CR blocker, so we can make incremental progress on CSS 2.x.