protocol extension: define what ‘additional characters’ mean

[andreastt]

The specifications talks about ‘additional characters’ when it refers to the vendor prefixes defined in CSS 2.1. The CSS spec does not define what these are, so perhaps we should do that in our spec.

An alternative, and perhaps more reusable approach, would be to define this more clearly in the WHATWG Compat spec or in Infra.

cc @jugglinmike