htmx is based on the ideas of hypermedia, you can read the first chapter in our book on the topic here:

https://hypermedia.systems/hypermedia-a-reintroduction/

HTMX is a layer on top of server-side rendered HTML that gives you near-SPA-levels of interactivity without having to build a standalone SPA.

HTMX is a polar opposite to Angular in terms of complexity, for one.