Well, HTTP does have etag ( https://en.wikipedia.org/wiki/HTTP_ETag ) but of course that would still require a request sending the known etag, to either get 304 not modified or the content. So how about a way to put the etags of assets into the header of the document loading them or something? Then the browser can decide if it wants to make that request.

And I also have the feeling that surely, just this exist, I just don't know about it ^^

Linking things by hash, other than being so very ugly, consider this case: you have this asset that changes every minute, and is several MB. If a users users your site too much, you will flush out out all other cached stuff with old versions of that file that will never get referenced again. That just strikes me as extremely wasteful, that is, you get a short term boost but even worse performance overall. If other sites do it too much, it will mean your own stuff will not even be cached when visitors come back.