Can't preload for type-switching <picture>s

[eeeps]

(copied over from whatwg/html#6400, at @annevk's reccomendation)

This was originally discussed when we were figuring out imgsrcset and imgsizes, but I encountered it again this week, and I thought it was worth documenting in its own issue.

Let's say I have the following <picture>

<picture>
  <source
    type="image/avif"
    srcset="a.avif"
  />
  <img
    src="a.jpg"
    alt="" />
</picture>

There's no way to preload just the AVIF, in browsers that support AVIF, and just the JPEG, in browsers that don't support AVIF. The best we can do right now is:

<link
  rel="preload"
  as="image"
  href="a.avif"
  type="image/avif"
/>

Which preloads the AVIF in browsers that support it, and doesn't preload the JPEG for anyone. Honestly this isn't the worst thing in the world, when preload is framed as an enhancement, but it is kind of a bummer that adopting a bleeding-edge image format forces a performance regression for older/non-supporting browsers.

[yoavweiss]

As the separate preload spec is now discontinued, whatwg/html#6400 is a better place to discuss this. Apologies for the noise!