I like that it still feels like html. I think that's it's biggest selling point.

You write:

  <div id="moo" />
  <form hx-put="/foo" hx-swap="outerHTML" hx-target="#moo">
     <input hidden name="a" value="bar" />
     <button name="b" value="thing">Send</button>
  </form>

Compared to (ChatGPT helped me write this one, so maybe it could be shorter, but not that much shorter, I don't think?):

  <div id="moo" />
  <form>
     <input hidden name="a" value="bar" />
     <button name="b" value="thing" onclick="handleSubmit(event)" >Send</button>
  </form>

  <script>
  async function handleSubmit(event) {
    event.preventDefault();
  
    // the form submit stuff
    const form = event.target.form;
    const formData = new FormData(form);

    const submitter = event.target;
    if (submitter && submitter.name) {
      formData.append(submitter.name, submitter.value);
    }

    // hx-put
    const response = await fetch('/foo', {
      method: 'PUT',
      body: formData,
    });

    / hx-swap
    if (response.ok) {
      const html = await response.text();
      // hx-target
      const target = document.getElementById('moo');
      const temp = document.createElement('div');
      temp.innerHTML = html;
      target.replaceWith(temp.firstElementChild);
    }
  }
  </script>

And the former just seems, to me at least, way way *way* easier to read, especially if you're inserting those all over your code.