Allow common js, css, and font libraries to be *installed*
The web abounds with popular js and css libraries that are constantly downloaded and sometimes cached by the browser. My suggestion is that MS extend the caching concept to make these libraries installable, in a manner similar to a plugin or extension.
Examples include jquery, Bootstrap, Foundation, Skel, and numerous icon fonts like font awesome, glyphicons, and so forth. These could be installed permanently by the user, with an option for automatic updates. For something like jquery, several different versions are probably necessary. Note that if this feature is implemented, there will be incentive for web developers to converge on a handful of major jquery versions instead of the plethora we see now: http://www.stevesouders.com/blog/2013/03/18/http-archive-jquery/
This will require that when IE sees a jquery or fontawesome URL, it check to see if it already has it installed and just load its local copy. There could be rules that specify when a jquery version is close enough vs when to go download it.
The whole idea of them being NOT installed is that the webmaster tests his site, with his own versions of the dependencies, and makes sure that works.
Making the website have to use some installed (unknown or at least possibly incompatible) version would be going backward.
I do agree that we could save some mS by not downloading this stuff, but most of the time, it is coming from a CDN anyway.