> it is a small js library ... This means that in theory you don't need js

I assume I'm not the only person left a little puzzled.

Do you mean "don't need JS" as in like, a full-fledged JS framework?

See the quickstart from htmx.org

    <script src="https://cdn.jsdelivr.net/npm/htmx.org@2.0.8/dist/htmx.min.js"></script>
      <!-- have a button POST a click via AJAX -->
      <button hx-post="/clicked" hx-swap="outerHTML">
        Click Me
      </button>

As a dev you get to write HTML, and need to learn about some new tag attributes.

Your browser is still running js.

They mean that you don't need to write JS, you can just add a script tag to your page

Sorry I know other people have responded already, but it means you don't have to write any client side javascript at all. You can just skip along with a server that returns html. Just throw the script tag in there, and you're done.

The magic of this is that it's pretty easy to make SPA-like webapps with no javascript or complex client side framework. You can write your server in python, rust, clojure, whatever. If you don't need a lot of state management, it's really simple and awesome.