Error handling for sizes attribute

[zcorpan]

#97 is about srcset, let's talk about sizes.

Pondering about it a bit, I think it makes more sense to use CSS rules for sizes than it does for srcset. sizes can contain media queries, and media="" already uses CSS rules in HTML. Also it's supposed to support things like calc or other things CSS supports in place of a <length>.

Right now, if sizes fails to parse, the attribute is ignored. Is that what we want?

Take this example from the spec:

<picture>
  <source sizes="(max-width: 30em) 100%, (max-width: 50em) 50%, calc(33% - 100px)"
          srcset="pic100.jpg 100w, pic200.jpg 200w, pic400.jpg 400w,
                  pic800.jpg 800w, pic1600.jpg 1600w, pic3200.jpg 3200w">
  <img src="pic400.jpg" alt="The president giving an award.">
</picture>

If the UA doesn't support calc, then it acts as if sizes was absent. Do we instead want to act as if sizes="(max-width: 30em) 100%, (max-width: 50em) 50%" (so that the browser at least does the right thing for 2 of the 3 layouts)? Or skip the whole source (so the author can specify a fallback source without using calc)? Something else?

[yoavweiss]

I think that skipping the whole source, so that a fallback sizes can be defined is better for future compat. In the "just use <hat you understand" option, authors cannot define a fallback for older browsers.

OTOH, if the author haven't defined a fallback source, older browsers will get a significantly worse experience with this option.

I guess it boils down to what is more probable. Do we think that authors are likely to define fallback content?

[zcorpan]

Realistically speaking, it seems unlikely that authors will include a fallback source. It seems more probable that they will avoid using calc until it is supported or use a polyfill. From that perspective it seems unnecessary to tweak the behavior.

[zcorpan]

@tabatkins in #86 says that invalid sizes will result in the source being skipped (although that's not what the current spec says).

[tabatkins]

Hm, that's a problem. It should skip things. I'll fix.