Require default length in sizes (authoring req)

[zcorpan]

See #253

since we require the sizes attribute now because defaulting to 100vw is unexpected, do we also want to require a default length for the same resaon?

maybe people will write media-conditions that are mutually exclusive, in which case a default length is not needed ? We could make it a parse error if the algorithm falls back to 100vw.

[zcorpan]

I'm leaning towards the simplest option, change the grammar to require the default length.

[yoavweiss]

If we're changing the grammar, it'd also change the processing model, not just the authoring requirements, right?

What should the browser do if the default length is not there?

[zcorpan]

No need to change the processing model.

[yoavweiss]

OK