&gt; If everyone simply linked the &quot;canonical&quot; version of jQuery (the CDN link is right on their site) then requiring jQuery will be effectively free because it will be in everyone&#x27;s cache.<p>It&#x27;s not, though.  I ran this experiment when I tried to get Google Search to adopt JQuery (back in 2010).  About 13% of visits (then) hit Google with a clean cache.  This is Google Search, which at the time was the most visited website in the world, and it was using the Google CDN version of JQuery, which at the time was what the JQuery homepage recommended.<p>The situation is likely worse now, with the rise of mobile.  When I did some testing on mobile browsing performance in early 2014, there were some instances where <i>two pageviews</i> was enough to make a page fall out of cache.<p>I&#x27;d encourage you to go to chrome:&#x2F;&#x2F;view-http-cache&#x2F; and take a look at what&#x27;s actually in your cache.  Mine has about 18 hours worth of pages.  The vast majority is filled up with ad-tracking garbage and Facebook videos.  It also doesn&#x27;t help that every Wordpress blog has its own copy of JQuery (WordPress is a significant fraction of the web), or for that matter that DoubleClick has a cache-busting parameter on all their JS so they can include the referer.  There&#x27;s sort of a cache-poisoning effect where every site that chooses <i>not</i> to use a CDN for JQuery etc. makes the CDN less effective for sites that do choose to.<p>[On a side note, when I look at my cache entries I just wanna say &quot;Doubleclick: Breaking the web since 2000&quot;.  It was DoubleClick that finally got&#x2F;forced me to switch from Netscape to Internet Explorer, because they served broken Javascript in an ad that hung Netscape for about 40% of the web.   Grrrr....]