It's a pretty core part of their design philosophy, possible the core.

https://htmx.org/essays/rest-explained/

So you can support JSON while still being REST. For example, Datastar supports merging in HTML, JS, JSON into the current view of a resource. They work together to keep the resource state unified versus polling. In general the Datastar way is...

1. make an MPA 2. each page is a resource 3. keep a stream open on the current state of that resource 4. ship, touch grass, repeat