DevToolbox

HTML Live Preview

Write HTML, CSS, and JavaScript with instant live preview in a sandboxed iframe. No setup required.

About This Tool

The HTML Live Preview is a free, browser-based code editor that lets you write HTML, CSS, and JavaScript and see the results instantly. It works like a lightweight alternative to CodePen or JSFiddle, but with one critical difference: all processing happens entirely in your browser. No code is sent to any server, making it safe for prototyping with sensitive markup or proprietary styles.

The editor is split into three panels — one each for HTML, CSS, and JavaScript — with a live preview that updates automatically 300 milliseconds after your last keystroke. The preview runs inside a sandboxed <iframe> with the sandbox="allow-scripts" attribute, meaning it can execute your JavaScript but cannot navigate the parent page or access cookies outside its context.

A built-in console output panel captures console.log, console.error, console.warn, and console.info calls from your script, so you can debug without opening the browser DevTools. Uncaught runtime errors are also captured and displayed with line numbers when possible.

You can switch between horizontal (editors on top, preview below) and vertical (editors on the left, preview on the right) layouts depending on your screen size and preference. Six preset templates — including a CSS Flexbox demo, a CSS Grid demo, a form layout, and an animation example — are provided so you can jump straight into experimenting. When you are done, copy the combined HTML document to your clipboard or download it as a standalone .html file.

How to Use

  1. Select a template from the dropdown or start from scratch.
  2. Write or paste your HTML in the HTML tab. The preview updates automatically after a short delay.
  3. Switch to the CSS tab to add styles, and the JS tab for JavaScript logic.
  4. Check the Console panel below the preview for console.log output and error messages.
  5. Toggle between Horizontal and Vertical layout using the layout button.
  6. Click Run to force a fresh preview, or Copy HTML to copy the full document.
  7. Use Download to save the combined HTML as a file, or Clear to reset all editors.
  8. Keyboard shortcut: Ctrl+Shift+C copies the combined HTML to your clipboard.

FAQ

Is my code sent to any server?

No. All rendering happens locally in your browser using a sandboxed iframe. Your HTML, CSS, and JavaScript never leave your machine.

Why does the preview use a sandboxed iframe?

The sandbox="allow-scripts" attribute ensures your JavaScript can run but cannot access the parent page, navigate away, or submit forms outside the iframe. This keeps the main application secure even if you accidentally paste malicious code.

How does the console panel work?

Inside the iframe, console.log, console.error, console.warn, and console.info are overridden to send messages to the parent page via postMessage. The console panel listens for these messages and displays them with color-coded labels.

Can I use external libraries or CDN links?

Not directly, because the sandboxed iframe does not have network access by default. However, you can paste the library source code directly into the JavaScript editor or include it inline via a <script> tag in the HTML panel.

How often does the preview refresh?

The preview is debounced by 300 milliseconds. It updates automatically 300 ms after your last keystroke in any editor. You can also click the Run button to force an immediate refresh.

Can I download my work?

Yes. Click the Download button to save a standalone .html file containing your HTML, CSS (inside a <style> tag), and JavaScript (inside a <script> tag). You can open this file directly in any browser.

What keyboard shortcuts are available?

Ctrl+Shift+C copies the combined HTML document to your clipboard. Standard text editing shortcuts (undo, redo, select all) work inside the editor textareas as usual.

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