What would it take to get the w3/html5 folks simply add a src-hash="$algo:$value" to any tag that can load remote resources? Seems like a low-impact way to sig

What would it take to get the w3/html5 folks simply add a src-hash="$algo:$value" to any tag that can load remote resources? Seems like a low-impact way to significantly boot the usefulness + security of CDN's. If the source page (requested over https, and presumedly not MITM'ed already) declares "I want to load that resource over there, and I expect it to hash to this value", then we get all the benefits of caching + trust that it has not been tampered.