Cross-site caching of javascript libraries

This is a proposal for extending the script tag of HTML.

The purpose of this proposal is to allow browsers to take advantage of the many great javascript
libraries that are now in common use (prototype, dojo, scriptaculous, e.t.c.) by many different sites
and minimize bandwidth – download times.

The idea is, since I already have (in my browser’s cache) the X javascript library (because site Y uses it),
why do I have to redownload it while visiting site Z ?

So, I propose adding a hash attribute to the script tag. A browsers behaviour will go like this:

• if a script tag doesn’t have a hash attribute, continue as normal.
• if there’s a hash attribute, check our global javascript cache.
  • If found, use library from cache.
  • Otherwise, download library and use it. Also, compute its hash, and if it matches the hash attribute add
    it in global cache.

So, what’s your opinion on this? Do you see any potential (security) problems?