Take for instance, common Javascript libraries hosted on CDNs. Every time you visit a jQuery based page where the js file is on a CDN you reveal to the

Take for instance, common Javascript libraries hosted on CDNs. Every time you visit a jQuery based page where the js file is on a CDN you reveal to the host of that CDN (e.g. Google) what website you're on Generally that only happens the first time you load the library: $ curl -I ajax.googleapis.com/ajax/libs/jquery/2.0.3/jquery.min.js ... Cache-Control: public, max-age=31536000 ... After that it should be in cache, and because lots of sites use jquery from ajax.googleapis.com it should stay there. (I work at Google on ngx_pagespeed, and sit next to the people who maintain the hosted libraries.)