TIL that you can use Scastie with Scala JS and, if one of the expressions returns an Element, it is rendered on the worksheet:

Quick example: https://scastie.scala-lang.org/toqnawY0QrKKwMDcHcAf2A

I need to work on the Minart APIs to make this use case a bit more ergonomic, but this looks pretty cool for demos.