It's pretty cool seeing demos on MDN1 have an "Open in CodePen" button. As in "Mozilla Developer Network", the massive and most comprehensive documentation for web technology out there. ❤
For example, the page for repeating-radial-gradient() has some neat demos like these:
One click and you can hop over to CodePen and play with that demo. Like all the integrations of this ilk, it uses our Post to Prefill API.