[mediaqueries][css-conditional] else

During the face to face, we talked about the fact setting default styles outside of a media query, then undoing then in the media query before applying new ones is painful, and that authors often want to do something like if/else.

I suggest we go with this syntax:

@media (something) {
 ...
 ...
 ...
 @else {
 }
}

@supports (foo:bar) {
 ...
 ...
 ...
 @else {
 }
}

I think putting the else inside the conditional rather than following it, is more consistent with how things work in CSS, since we don't currently have a concept of associating two separate @rules.

The transition phase isn't too nice, as this isn't really usable until all browsers support it, but I think this is going to be the case for any syntax we can pick for this feature.

Thoughts?

- Florian