HTMX is a small (14kb) JavaScript library that lets you build interactive web applications by adding attributes to HTML — no JavaScript required in your code. It sends AJAX requests, swaps HTML fragments returned from the server, and handles WebSocket, SSE, and history.
Example: <button hx-post="/like" hx-swap="outerHTML">Like</button> — clicking sends a POST to /like, and the server-rendered response replaces the button. No React, no build step.
HTMX is the flagship of the "hypermedia-driven applications" movement, championed by developers moving back to server-rendered HTML for simpler stacks. Great for CRUD apps, admin panels, and content-heavy sites. Less great for highly interactive apps with complex client-side state.