Isn't the problem more that the browser can't be given
     multiple sources and types and allow it to chose what
     it believes is best?

How would this look? If you simply did:

    <img src-options="image.jpg
                      image.webp">

There's no way for the browser to know which one it wants without downloading all of them, because extensions aren't special. But I guess we could extend srcset to let you specify content type:

    <img srcset="medium.jpg  1000w image/jpeg,
                 medium.webp 1000w image/webp,
                 large.jpg   2000w image/jpeg,
                 large.webp  2000w image/webp">