Many of you have asked for live code examples that allow you to tweak the code and observe what happens. We agree that a feature like this would be of huge educational value and are actively working on providing it.
We plan to use a hosted (and probably customized) version of dabblet, an open source project built by yours truly. We decided to use dabblet because it's a mature project that has been extensively tested in the community by a couple thousand users every day for almost a year, and because of its unique features, such as its CSS value previewers.
This work will take some time, but we are confident that when it’s out you will agree with us that it was absolutely worth it.