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.
I'm leaning towards the simplest option, change the grammar to require the default length.
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?
No need to change the processing model.
Require a default length in sizes. Fixes #256