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UPDATE: The CodePen Bookmarklet is deprecated. We no longer maintain it and can't support it.
Bookmarklets, in general, are going away thanks to CSP (Content Security Policy) on the web (a good thing). That, plus the difficult technical debt of needing to update the bookmarklet against third-party changes, made it too impractical to maintain. We'll leave this post up for posterity.
We'd recommend checking out the prefill api, in the case you wanted to build something to replicate any of its functionality.
This is our first release of it. We've tested it on a bunch of sites and it works pretty good, but if you find some that it doesn't work on, let us know in the comments below. Thanks for playing!