[mediaqueries] @media not (unsupported-media-feature) v.s. @media not (unsupported + syntax)

MQ4 adds a <general-enclosed> term to its grammar that accepts almost 
anything and evaluates to false. This is to support future extensions.

This means that a query like `not (unsupported + syntax)` evaluates to true.

However, something like `not (unsupported-media-feature)` with the 
normal <media-feature> syntax evaluates to false:

> An unknown <mf-name> or <mf-value>, or disallowed <mf-value>, must
> make the entire <media-query> be replaced by not all.


This is inconsistent. Should one of these two cases be changed to match 
the other?

-- 
Simon Sapin