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While creating a Pen, you can link out to external resources like JS and CSS files. For JS, we've long had a "typeahead" feature where you can start typing the name of a common JS resource and it will provide a menu of options for that resource. For instance, you can type "under", referencing the popular library Underscore.js, and you'll be able to select a URL to a CDN where underscore is hosted.
Now, we've added a couple of new typeahead options for the CSS external resources: Bootstrap and Foundation. These are super popular CSS frameworks. People use them in a variety of ways. Some for rapid prototyping. Some for a reliable foundation to build out from. Some just to slap some style on a simple project that could use it.