How should filtering by stability work?

[SebastianZ]

With #212 in place, there is an option to filter the specs by stability. A specification is marked as stable if it is listed anywhere in one of the CSS snapshots or is expected to not change (much) anymore. Otherwise it is marked as experimental.

That was the simplest solution I could come up with. Though there may be other ways to define the stability of the specs.

Specific questions:

• Should the definition of spec. stability be more fine-grained? E.g. the CSS snapshots define different types of stability.
• Should the definition of stability be extended to individual features? If so, how would that look like?
• How should the filters be defined regarding the stability?

This issue overlaps with #220 to some extent.

Sebastian