[css-images-4] multiple <image-set-option> with the same resolution

[jonathantneal]

The specification describes a scenario where image-set() functions are invalidated.

Every <image-set-option> in a given image-set() must have a different <resolution>, or else the function is invalid.
— https://drafts.csswg.org/css-images-4/#image-set-notation

This does not seem to reflect existing implementations. Neither Safari or Chrome fail to display an image in the following scenario:

html {
  background-image: -webkit-image-set(
    url(https://cloudfour.com/examples/image-set/assets/test.png) 1x,
    url(https://cloudfour.com/examples/image-set/assets/test-hires.png) 1x
  );
}

Both will use the last applicable <image>. Should this be reflected in the specification, or should we file browser bugs?

[csnardi]

I think the behavior as specified makes sense, as currently a collection of image-set options with the same resolution wouldn't make much sense. If other factors than resolution were considered per the inline issue in the spec, it might make sense to specify two images to be used at a particular resolution, as one could be "better" than the other. But in terms of the logic that Safari/Chrome implements, I would believe that it would just choose the first image for that resolution.

I plan on matching the spec regarding this behavior when unprefixing image-set in Chrome (hopefully soon!).

[tabatkins]

Cool, knowing that Chrome intends to match the spec inclines me to keep it the way it is.

(When I specced the feature I didn't test WK's behavior, I just wrote what I assumed made the most sense. If browsers don't want me to do that, they should actually spend a modicum of effort speccing things before shipping them publicly.)