Many HTTP requests are sent repeatedly by browsers on almost every page load, such as requesting the jQuery .js file etc. Since these are already used on too many sites doesn't modern browsers keep a cache for this?

I am thinking of a system where the browser has a cached copy of the .js file used very very frequently. On a new request for the .js file, it sends the server a request for a hash of the .js file (provided the server can reply to that) and compares the returned hash with the cached copy's hash... rest is intuitive.