Usually a browser restricts the number of simultaneous connections to a single domain (depending on the browser 2-8 connections). That’s why Yahoo recommends to use a Content Delivery Network (CDN) to decrease the number of connections to the same domain.

Lately I’ve been experimenting with CloudFlare’s CDN. They host JavaScript, CSS and even the images. By using the protocol relative (//) they’re leveraging http, https and even spdy.

This type of hosting is also great with online editors as JS Fiddle.