I've just run my own analysis on the HTTP Archive data, and the fragmentation issues are very real. The most popular URL used to load jQuery was: http://ajax.g

I've just run my own analysis on the HTTP Archive data, and the fragmentation issues are very real. The most popular URL used to load jQuery was: http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min... ...and it was used by just 2.7% (945) of the 35,204 pages in the dataset. Note that it's not just version fragmentation - you have to take protocol into account too as browsers cache HTTP and HTTPS separately. The next most popular was: http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min... ...used by 1.3% (460) of pages, followed by: http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min... ...used by 0.8% (285) of pages. At this point there really isn't much of a debate; unless you have evidence to the contrary (e.g. all our visitors come from Facebook, and Facebook use the same version of jQuery as we do) using Google's CDN to load jQuery isn't likely to benefit the majority of your first-time visitors.