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