As mentioned, htmx will attach a `HX-Request: true` header to the request[1]. The server can check this header and either return a partial for swapping, or a full page/redirect like in good old days. Same with any request. This is one way of how htmx provides "progressive enhancement". Sure, this may not be as transparent, as other JS-first frameworks implement it, but it's not complex at all.

The only thing that might cause trouble is non-standard (as in HTML standard) HTTP methods, which basically means any method other than GET and POST, I admit that. However, the fact that these methods are not supported even in HTML5 is a huge miss.

[1]: https://htmx.org/reference/#request_headers