Srcset implementations do not take width as a fallback when sizes is not present

[yoavweiss]

While looking into #4968, I noticed that HTML's parsing a sizes attribute is taking the image element's width into account as the fallback, in case sizes is not present.
This used to make sense, before the changes made in WICG/intrinsicsize-attribute#16 (comment)
Since width no longer implies absolute pixel values, it no longer does, and I think we should remove that.

Looking at the different implementations (test page - needs to be tested on 1 DPR screen), none of them treats width as the fallback value, meaning we can easily remove that from the spec.

@zcorpan @annevk @chrishtr - thoughts?

[zcorpan]

As far as I can tell, it's implemented in chromium/webkit/gecko. Compare with no width or sizes attribute, which defaults to 100vw.

[zcorpan]

This used to make sense, before the changes made in WICG/intrinsicsize-attribute#16 (comment)
Since width no longer implies absolute pixel values, it no longer does, and I think we should remove that.

I think that change doesn't influence much for this behavior. The width attribute could be overridden with CSS before just like now. It's also a fallback that is only used in invalid documents, since the sizes attribute is required when srcset uses w.

[zcorpan]

Hmm, wait. I was confused in my first comment. A clearer test is <img width=400 style="width: auto" ...>, where the intrinsic size from sizes is used for rendering, which is 100vw in chromium/webkit/gecko.

OK, since it's not implemented, removing it seems fine.

[annevk]

Agreed.