A small suggestion for the site would be a more realistic view port for testing mobile. I enjoy the ability to adjust the width to see how it looks on the various screen widths however another area of importance is the height. I always want to see if the section I have created will fit properly on a screen without scrolling. If I have an action button (next, read more, join, etc) I really want that button to display within the same screen as the call to action text. I find I am always going into Chrome to test the various device viewports and ensuring the mobile layout is where I want it. The full page view that you provide now is useful as well, however, it would be nice to be able to test the height when needed.
Just a thought,