URLs without protocols get "file:" prepended when their document is
loaded as a file. Make sure the docs can always be opened by
specifying a protocol.
Twitter's widget library[1] provides an async snippet to inject the
script tag later. Since widgets.js came before the local JS, if
twitter.com took long to respond it would prevent subsequent
synchronous script tags.
[1] https://dev.twitter.com/docs/tfw-javascript
jQuery.com uses Google Analytics and so sets a cookie when you visit
the site. When you request jquery.js from its CDN, code.jquery.com,
you take a hit from sending your previous GA cookie in the request.
Google Hosted Libraries[1] and googleapis.com never set cookies.
[1] https://developers.google.com/speed/libraries/devguide
