Meta: figure out CSS Colors Level 4 support plan

[Dianliang233]

rebeccapurple (#663399) is a named color added in CSS Color Module Level 4 to honor web pioneer Eric Meyer. Although Level 4 is not a formal W3C Recommendation & rebeccapurple is not in the SVG 1.1 spec, it is a Candidate Recommendation Draft and all browsers have supported this value since 2015. Therefore, this color should probably be added, and tests/definitions.py should migrate to Level 4.

[ubernostrum]

I don't yet have a plan for when to implement CSS Colors Level 4, or how much of it to implement, given the way the spec and its development work now. So I'd like to do this as part of a Colors Level 4 update release, but that requires figuring out what the Colors Level 4 support will look like. I'm going to rename this issue to indicate that, and leave it open until Colors Level 4 support is worked out.