But really these kind of playgrounds are at least a cousin, if not a direct descendant, of the immediate feedback principle that underlies everything from a REPL to the kind of systems (Symbolics, etc) that allow you to edit running code. Their potential is beautifully illustrated in Brett Victor's famous 'Inventing on Principle' talk: https://www.youtube.com/watch?v=PUv66718DII
Three areas that I'd love to see playgrounds mature beyond teaching beginners CSS:
1) Integrated directly into developer documentation. You can see some of that in systems that are documenting services -- for example, the Google Sheets API documentation has a 'Try this Method' box that lets you post a request to the service on the same page that describes it, but what I really want is something like that when I'm reading the man page of a destructive disk command, or a complex database query, or a low-level C graphics library. That'll require a playground wired to something as big as a VM, maybe.
2) Integrated into textbooks: for the last couple years the astonishing posts from Bartosz Ciechanowski ([Sound](https://ciechanow.ski/sound/), [Bicyle](https://ciechanow.ski/bicycle/)) have demonstrated just how much better a textbook could be if you could interactively play with the concepts you're reading about. Unlike dev docs, the thing you're interacting with doesn't even have to be "real".
3) Most ambitiously: integrated into the input form of bug/issue tracking tools. This is the opposite of play: it means a running system would need to be in cahoots with a playground, so that mature apps could let you record and playback what you saw as a user, allow you to edit irrelevant parts of input and output out and submit a complete here-look-see-it-is-busted asset for analysis. A Stackoverflow of the future would be orders of magnitude more interesting if each question was posed as a broken playground.