The geneal idea of HTMX is that your HTML will be rendered by the backend — à la Server Side Rendering.
To me this phrase makes no sense, what's the thought process behind this meaning of "render"? The only place HTML is "rendered" is in a browser (or in a user agent, if you prefer).
You could "render" it to html with pandoc, then serve the html from disk by a static web server.
This would be "build time" html - it's html before it's read by the server.
Then you could setup a cgi script, php-script or an application server that converted markdown to html - then sent it to the client. This would be server-side rendering.
Finally, you could send some html+javascript to the client that fetch the raw markdown, then generates html on the client. That would be client side rendering.