[whatwg] img srcset / css image-set issues

[Tab Atkins Jr.]

On Tue, Aug 21, 2012 at 12:31 PM, Tab Atkins Jr. <jackalmage at gmail.com> wrote:
> On Tue, Aug 21, 2012 at 12:20 PM, John Mellor <johnme at chromium.org> wrote:
>> CSS image-set and HTML img srcset are getting their first implementations
>> but both APIs have serious shortcomings. We should fix them before it's too
>> late:
>>
>> 1. Neither is of any use for flexibly-sized images.
>> 2. srcset isn't as smart/intuitive as image-set.
>> 3. image-set is less flexible than srcset.
>>
>> I'll go through these in turn, in decreasing order of importance (sorry
>> about the length, but this is a complex topic and I've tried to avoid
>> ambiguity).
>>
>> (This email is cross-posted to whatwg and www-style, since this is of equal
>> relevance to HTML and CSS).
>>
>> 1. Neither is of any use for flexibly-sized images.
>
> This is what Media Queries and the similar 'w' and 'h' tokens in
> @srcset are for.  You can create one pair of 1x/2x images for one size
> of screen, and another pair for another size of screen.
>
>
>> 2. srcset not as smart/intuitive as image-set.
>
> The algorithms are meant to be the same.  If you read the HTML
> algorithm carefully, you'll notice that, after you've eliminated the
> images that violate the 'w' or 'h' constraints, the browser can choose
> *any* of the remaining images, based on whatever criteria it wants.
> This happens in two places: step 17 is very explicit in saying "UAs
> can do what they want"; step 21 is more implicit, as the "nominal
> pixel density" is a UA-defined value and can be literally anything.
> (There was previously a note in the spec saying this directly, but it
> appears to have been removed.)
>
> Ideally, I'd be able to just refer to the HTML algorithm, but it's far
> too specialized for @srcset for me to actually use.  Instead, I plan
> to include a note pointing to @srcset and indicating that it should be
> identical.
>
> There is currently a behavior difference between HTML and CSS - CSS
> currently does fallback to less-good images if the first doesn't load,
> while HTML doesn't.  I'm going to harmonize the two, either by getting
> HTML to do fallback or changing CSS to not do it.
>
>
>> 3. image-set is less flexible than srcset.
>
> It's not.  The extra things that @srcset can do are precisely
> identical to just using Media Queries.
>
> ~TJ