This way you can set JPG XL as the default, and a lower quality normal JPG file as the backup (maybe by setting both to the same file size, lol). And then separately give the user a notice (based on feature testing) that their browser doesn’t support JPG XL and that they should request it
Here’s some HTML only tricks to serve a default image plus a fallback image;
https://stackoverflow.com/questions/980855/inputting-a-default-image-in-case-the-src-attribute-of-an-html-img-is-not-vali#980910
This way you can set JPG XL as the default, and a lower quality normal JPG file as the backup (maybe by setting both to the same file size, lol). And then separately give the user a notice (based on feature testing) that their browser doesn’t support JPG XL and that they should request it