not sure it counts as "HyperCard" but for environments there's p5js.org (https://editor.p5js.org/)
> I wish there were an offline environment like that built into firefox. You can bring up a javascript console, but it's not really a friendly development environment. And something like visual studio code is capable but large and complicated.
That seems like a day project in electron. At least to MVP. Use CodeMirror or Monaco. As long as you pass the right options you can run the user's code in an iframe in a separate process. That way you can kill it even if it has an infinite loop.
> I wish there were an offline environment like that built into firefox. You can bring up a javascript console, but it's not really a friendly development environment. And something like visual studio code is capable but large and complicated.
That seems like a day project in electron. At least to MVP. Use CodeMirror or Monaco. As long as you pass the right options you can run the user's code in an iframe in a separate process. That way you can kill it even if it has an infinite loop.