It will select the first supported format, webp in the example below.

  <picture>
    <source srcset="foo.webp" type="image/webp">
    <source srcset="foo.jxr"  type="image/vnd.ms-photo">
    <img src="foo.jpg" alt="" width="100" height="150">
  </picture>

https://html.spec.whatwg.org/dev/images.html#image-format-ba...

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About the Accept header, currently they include `image/avif` so I have no reason to believe that `image/jxl` won't be added.