How to make the browser choose between and

Question

GIFs are heavy (xx mb), so we convert them to mp4 and webp. How to make the browser choose <img webp> or fallback to <video mp4> if he does not support webp?

I know we can use <source> in one <video> or in one <picture>, but how to make the browser choose between one <img webp> and fallback into <video mp4> if needed?

Is there a way to put the webp in the <video>? or a mp4 in <img>?

Regards