[css-content] URL fallbacks in 'content' should require type() or format()

[zcorpan]

https://drafts.csswg.org/css-content/#example-1a710936

/* Replace <logo> elements with the site’s logo, using a format
 * supported by the UA */
logo { content: url(logo.mov), url(logo.mng), url(logo.png), none; }

I think this is bad. In order for the fallback to work at all, the UA needs to fetch each resource in order and try to decode it, and stop when it has found something it can decode. This is like <video><source> when not using the type attribute, and I think it is bad both for page loading performance and for increased implementation complexity. We carefully avoided it when designing <picture>'s processing model.

If we want this to work, I think a type() or format() function (or some such) to go with each URL should be provided, so the UA can skip resources it knows it doesn't support right away. If it isn't provided, then the UA should load the given resource and if it finds it doesn't support it, it should not try to load the next item; it should skip straight to the next non-url fallback (none above).

cc @fantasai @tabatkins @yoavweiss

[zcorpan]

Or maybe this should be handled by image-set() instead.

[tabatkins]

Not image-set(), that's for multiple resolutions. image() is intended for fallback, and I have a long-standing request to add type() to its arguments for precisely this reason.

(Well, image-set() should also have it, because <picture> can do so, but that's just to ignore some values, it doesn't do fallback based on that.)

[LeaVerou]

image() has been deferred to Level 4. I propose deferring type() in image-set() to Level 4 as well. Does that work for you, @zcorpan?

[fantasai]

Sure, it's still an issue against css-content-3 though.

[zcorpan]

Deferring type() to later is OK.

I believe the normative text already does not do fallback, like I requested in OP.

If an <image> represents an invalid image, the user agent must do one of the following:

So I think to resolve this issue it should be enough to fix the non-normative text that talks about fallback.

[LeaVerou]

So I think to resolve this issue it should be enough to fix the non-normative text that talks about fallback.

Not sure what change you're referring to. I can't find any text that mentions fallback? Could you elaborate?

[zcorpan]

See link in OP

[LeaVerou]

So the only edits required are in css-content?