htmx is a very different way of developing your web application. You can define a lot of behaviour inside your HTML with the new attributed added by htmx. This allows you to build an interactive website without using any JavaScript. You do need a REST API that returns HTML though.
For more information about HTMX you can read the htmx docs.
Cool concept, especially with making pure HTML feel responsive, but API endpoints returning HTML is definitely impractical for large applications. I wonder if XSS is a concern by default.