This is pretty cool, great job! I know this is only a cheatsheet, which implies simple, but one idea that I think might be useful would be to add something to allow an end-user to resize the preview pane. This would allow more complete "previewing" of responsive items, eg. "navbar-toggleable-*" which collapses the a navbar down to a hamburger when the width is small.

Once again though, nice work! :)