Idea for browser makers - persistent cache javascript libraries

I wanted to get this idea out there, though I'm sure I'm not the first. Browser makers should set up a way to persistently store javascript libraries in the browser. Like jQuery, Dojo, YUI, etc. (User could add as many as they wanted) and then when the browser hits a site that references that library, it uses the downloaded version instead of the remote one - make the apps much faster to download and use...

What do you say browser makers? What do you think of this idea?