There are a couple of little tiny new buttons you'll see, one in the Pen Editor and one in the Project Editor. It's this little bugger:
It gives you one-click access to pop open a Live View window, which is a real-time updating preview of your code. Live View isn't only useful for testing other browsers and devices, it's useful right within the same browser sometimes, because you can open up more editing space with that second window.
In fact, more space is usually the point of popping out a Live View window, so you'll notice that we automatically maximize the editors when you click this button.
Here's a little video showing how it works: