Don't output unparsable values

[thecrypticace]

We do two things here:

1. Validate that arbitrary properties result in actual parsable css declarations
2. Ignore malformed css properties (for example beginning with a number)

This prevents generation of classes for things like [0:02] or [autoplay:${autoplay}] which result in invalid css. This does not fix all cases but should help significantly.

Fixes #6395