htmx Expert

This skill provides comprehensive guidance for htmx development, the library that extends HTML to access modern browser features directly without JavaScript.

Core Philosophy

htmx represents a paradigm shift toward hypermedia-first web development. Instead of treating HTML as a presentation layer with JSON APIs, htmx extends HTML to handle AJAX requests, CSS transitions, WebSockets, and Server-Sent Events directly. Servers respond with HTML fragments, not JSON.

When to Use This Skill

• Implementing htmx attributes and interactions
• Building hypermedia-driven applications
• Debugging htmx request/response cycles
• Converting SPA patterns to htmx approaches
• Understanding htmx events and lifecycle
• Configuring htmx extensions
• Implementing proper security measures

Core Attributes Reference

HTTP Verb Attributes

Request Control

• hx-trigger: Customize when requests fire

  • Modifiers: changed, delay:Xms, throttle:Xms, once
  • Special triggers: load, revealed, every Xs
  • Extended: from:<selector>, target:<selector>

• hx-include: Include additional element values in request

• hx-params: Filter which parameters to send (*, none, not <param>, <param>)

• hx-headers: Add custom headers (JSON format)

• hx-vals: Add values to request (JSON format)

• hx-encoding: Set encoding (multipart/form-data for file uploads)

Response Handling

• hx-target: Where to place response content

  • Extended selectors: this, closest <sel>, next <sel>, previous <sel>, find <sel>

• hx-swap: How to insert content

  • innerHTML (default), outerHTML, beforebegin, afterbegin, beforeend, afterend, delete, none
  • Modifiers: swap:Xms, settle:Xms, scroll:top, show:top

• hx-select: Select subset of response to swap

• hx-select-oob: Select elements for out-of-band swaps

State Management

• hx-push-url: Push URL to browser history
• hx-replace-url: Replace current URL in history
• hx-history: Control history snapshot behavior
• hx-history-elt: Specify element to snapshot

UI Indicators

• hx-indicator: Element to show during request (add htmx-indicator class)
• hx-disabled-elt: Elements to disable during request

Security & Control

• hx-confirm: Show confirmation dialog before request
• hx-validate: Enable HTML5 validation on non-form elements
• hx-disable: Disable htmx processing on element and descendants
• hx-sync: Coordinate requests between elements

Implementation Patterns

Basic AJAX Pattern

<button hx-get="/api/data"
        hx-target="#result"
        hx-swap="innerHTML">
  Load Data
</button>
<div id="result"></div>

Active Search

<input type="search"
       name="q"
       hx-get="/search"
       hx-trigger="keyup changed delay:300ms"
       hx-target="#search-results">
<div id="search-results"></div>

Infinite Scroll

<div hx-get="/items?page=2"
     hx-trigger="revealed"
     hx-swap="afterend">
  Loading more...
</div>

Polling

<div hx-get="/status"
     hx-trigger="every 5s"
     hx-swap="innerHTML">
  Status: Unknown
</div>

Form Submission

<form hx-post="/submit"
      hx-target="#response"
      hx-swap="outerHTML">
  <input name="email" type="email" required>
  <button type="submit">Submit</button>
</form>

Out-of-Band Updates

Server response can update multiple elements:

<!-- Main response -->
<div id="main-content">Updated content</div>

<!-- OOB updates -->
<div id="notification" hx-swap-oob="true">New notification!</div>
<span id="counter" hx-swap-oob="true">42</span>

Loading Indicators

<button hx-get="/slow-endpoint"
        hx-indicator="#spinner">
  Load
</button>
<img id="spinner" class="htmx-indicator" src="/spinner.gif">

CSS for indicators:

.htmx-indicator {
  opacity: 0;
  transition: opacity 200ms ease-in;
}
.htmx-request .htmx-indicator {
  opacity: 1;
}

Server Response Patterns

Return HTML Fragments

Server endpoints return HTML, not JSON:

# Flask example
@app.route('/search')
def search():
    q = request.args.get('q', '')
    results = search_database(q)
    return render_template('_search_results.html', results=results)

Response Headers

htmx recognizes special headers:

Detect htmx Requests

Check HX-Request header to differentiate htmx from regular requests:

if request.headers.get('HX-Request'):
    return render_template('_partial.html')
else:
    return render_template('full_page.html')

Events

Key Events

Event Handling

Using hx-on*:

<button hx-get="/data"
        hx-on:htmx:before-request="console.log('Starting...')"
        hx-on:htmx:after-swap="console.log('Done!')">
  Load
</button>

Using JavaScript:

document.body.addEventListener('htmx:configRequest', function(evt) {
  evt.detail.headers['X-Custom-Header'] = 'value';
});

Security Best Practices

1. Escape All User Content: Prevent XSS through server-side template escaping
2. Use hx-disable: Prevent htmx processing on untrusted content
3. Restrict Request Origins:

   htmx.config.selfRequestsOnly = true;
   

4. Disable Script Processing:

   htmx.config.allowScriptTags = false;
   

5. Include CSRF Tokens:

   <body hx-headers='{"X-CSRF-Token": "{{ csrf_token }}"}'>
   

6. Content Security Policy: Layer browser-level protections

Configuration

Key htmx.config options:

htmx.config.defaultSwapStyle = 'innerHTML';
htmx.config.timeout = 0; // Request timeout (0 = none)
htmx.config.historyCacheSize = 10;
htmx.config.globalViewTransitions = false;
htmx.config.scrollBehavior = 'instant'; // or 'smooth', 'auto'
htmx.config.selfRequestsOnly = false;
htmx.config.allowScriptTags = true;
htmx.config.allowEval = true;

Or via meta tag:

<meta name="htmx-config" content='{"selfRequestsOnly":true}'>

Extensions

Loading Extensions

<script src="https://unpkg.com/htmx-ext-<name>@<version>/<name>.js"></script>
<body hx-ext="extension-name">

Common Extensions

• head-support: Merge head tag information across requests
• idiomorph: Morphing swaps (preserves element state)
• sse: Server-Sent Events support
• ws: WebSocket support
• preload: Content preloading
• response-targets: HTTP status-based targeting

Debugging

Enable logging:

htmx.logAll();

Check request headers in Network tab:

• HX-Request: true
• HX-Target: <target-id>
• HX-Trigger: <trigger-id>
• HX-Current-URL: <page-url>

Progressive Enhancement

Structure for graceful degradation:

<form action="/search" method="POST">
  <input name="q"
         hx-get="/search"
         hx-trigger="keyup changed delay:300ms"
         hx-target="#results">
  <button type="submit">Search</button>
</form>
<div id="results"></div>

Non-JavaScript users get form submission; JavaScript users get AJAX.

Third-Party Integration

Initialize libraries on htmx-loaded content:

htmx.onLoad(function(content) {
  content.querySelectorAll('.datepicker').forEach(el => {
    new Datepicker(el);
  });
});

For programmatically added htmx content:

htmx.process(document.getElementById('new-content'));

Common Gotchas

1. ID Stability: Keep element IDs stable for CSS transitions
2. Swap Timing: Default 0ms swap delay; use swap:100ms for transitions
3. Event Bubbling: htmx events bubble; use event.detail for data
4. Form Data: Only named inputs are included in requests
5. History: History snapshots store innerHTML, not full DOM state

Development Environment Requirements

htmx Requires HTTP (Not file://)

htmx will NOT work when opening HTML files directly from the filesystem (file:// URLs). This causes htmx:invalidPath errors because:

• Browsers block cross-origin requests from file:// URLs
• htmx needs to make HTTP requests to endpoints

Solution: Always serve htmx applications via HTTP server:

# Simple Python server (recommended for development)
python3 -m http.server 8000

# Or create a custom server with API endpoints
python3 server.py

Minimal Development Server Pattern

For htmx examples and prototypes, create a simple Python server that:

1. Serves static files (HTML, CSS, JS)
2. Provides API endpoints that return HTML fragments

from http.server import HTTPServer, SimpleHTTPRequestHandler
from urllib.parse import urlparse, parse_qs

class HtmxHandler(SimpleHTTPRequestHandler):
    def do_GET(self):
        path = urlparse(self.path).path

        if path.startswith("/api/"):
            # Return HTML fragment
            self.send_response(200)
            self.send_header("Content-Type", "text/html")
            self.end_headers()
            self.wfile.write(b"<div>Response HTML</div>")
        else:
            # Serve static files
            super().do_GET()

HTTPServer(("", 8000), HtmxHandler).serve_forever()

Practical Implementation Lessons

Loading Indicators with CSS Spinner

Use CSS-only spinners instead of image files for better performance:

<button hx-get="/api/slow"
        hx-indicator="#spinner">
    Load
    <span id="spinner" class="spinner htmx-indicator"></span>
</button>

<style>
.htmx-indicator { display: none; }
.htmx-request .htmx-indicator { display: inline-block; }

.spinner {
    width: 20px;
    height: 20px;
    border: 2px solid #f3f3f3;
    border-top: 2px solid #3d72d7;
    border-radius: 50%;
    animation: spin 1s linear infinite;
}
@keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(360deg); }
}
</style>

Input Search with Proper Trigger

Use input changed instead of keyup changed for better UX (catches paste, autofill):

<input type="search"
       name="q"
       hx-get="/api/search"
       hx-trigger="input changed delay:300ms, search"
       hx-target="#results">

The search trigger handles the search input’s clear button (X).

Self-Targeting with Polling

For elements that replace themselves (polling), use hx-target="this":

<div hx-get="/api/time"
     hx-trigger="load, every 2s"
     hx-target="this"
     hx-swap="innerHTML">
    Loading...
</div>

Row Updates with closest

For list items where each row has its own update button:

<li id="item-1">
    <span>Item 1</span>
    <button hx-get="/api/update-item/1"
            hx-target="closest li"
            hx-swap="outerHTML">
        Update
    </button>
</li>

Server returns complete <li> element with new htmx attributes intact.

Event Attribute Syntax

The hx-on:: syntax uses double colons for htmx events:

<!-- Correct -->
<button hx-on::before-request="console.log('starting')">

<!-- Also correct (older syntax) -->
<button hx-on:htmx:before-request="console.log('starting')">

Combining Multiple Triggers

Separate triggers with commas:

<div hx-get="/api/data"
     hx-trigger="load, every 5s, click from:#refresh-btn">

Form POST with Loading State

Combine hx-indicator and hx-disabled-elt for complete UX:

<form hx-post="/api/submit"
      hx-target="#result"
      hx-indicator="#spinner"
      hx-disabled-elt="find button">
    <input name="email" required>
    <button type="submit">
        Submit
        <span id="spinner" class="spinner htmx-indicator"></span>
    </button>
</form>

Additional Resources

For detailed reference, consult:

• Official docs: https://htmx.org/docs/
• Attributes reference: https://htmx.org/reference/
• Examples: https://htmx.org/examples/