I recently dug up an old project in an attempt to improve on it. It's a code sandbox for playing around with HTMX in the browser, that runs a mock server within the sandbox iframe. The server "framework" is loosely based on Django, so if you're familiar with Django, you'll immediately understand what's going on.
I recommend clicking through the examples.
Github repo: https://github.com/lassebomh/htmx-playground
Probably my favorite part is the lack of HTMX specific code. It's designed to mimic the client and server, but really nothing else. In principle, this means that it is agnostic to whatever frontend framework is being used.
Known problems: Limited mobile support, Ace Editor (should just be Monaco) and lack of proper error outputs.
Feel free to give feedback, suggestions or questions.
I learned a lot when making it, and I hope you'll something about HTMX! Happy tinkering.