in the specification, JPEG and PNG are allowed for screenshots, but this seems a little constraining now. Since Firefox and Chromium both support WebP (and most of GNOME does, too), it is a pretty appealing image format that can compress better than either of the other two in lossy and lossless modes both.

I would like to start moving web sites over to WebP-only, but the screenshots in the AppStream specification requiring me to use PNG is constraining to this goal.