If I am not mistaken, last time I looked at it I think I saw that HTMX has the ability to declare handlers. So that if you instead want to provide Json, one of those handlers could transform/handle it client-side. I think, haven't tried it myself.

Any backend that can spit out html fragments should works. The backend doesn't have to be aware of htmx. Heck, the response doesn't have to be a fragment either because you can just select which part of the response to update the dom with using the hx-select attribute.