I think the CDN alternative would be:

Site 1 sends <script src="/foo/jquery.js" hash="SHA3:12345...">. Your browser hasn't cached this file, so it downloads jquery.js, verifies the hash and caches the contents. Site 2 sends <script src="/bar/jquery-2.5.js hash="SHA3:12345...">. The browser finds that the hash matches the cached jquery.js and loads that instead of downloading the script again.

The scripts could still be served from CDNs, but it wouldn't have to be the same one to have a cache hit. Popular libraries like jQuery would have so many hits that a CDN might not even be worth the effort. Actually the concept is so simple, it's surprising that this hasn't already been implemented unless there is a security issue that I'm not seeing.