Development Tools
Live HTML Previewer — Real-Time HTML, CSS & JavaScript Testing
Preview HTML, CSS, and JavaScript live in a sandboxed iframe with mobile, tablet, and desktop viewport presets. Free, auto-updates as you type.
TL;DR: Write HTML, CSS, and JavaScript in three tabbed editors, and see the combined result render live in a sandboxed iframe with a 300ms auto-update debounce. Switch between Mobile (375×667), Tablet (768×1024), and Desktop (1920×1080) viewport presets, auto-scaled to fit your screen.
Reviewed by FYN Tools Editorial · Guides ·
Privacy & processing: 100% client-side browser processing — your code renders inside a sandboxed iframe on your own device; nothing is sent to a server.
Live HTML Previewer
Preview HTML, CSS, and JavaScript live in a sandboxed iframe with mobile, tablet, and desktop viewport presets. Free, auto-updates as you type.
- Inputs
- HTML, CSS, and JavaScript code entered in three separate tabs
- Outputs
- A live-rendered preview in a sandboxed iframe, viewable at three responsive viewport presets, plus a copyable combined HTML document
- Processing
- Client-side (browser, sandboxed iframe)
- Limits
- The preview iframe uses sandbox="allow-scripts allow-same-origin", so scripts requiring top-level navigation or certain cross-origin behaviors may be restricted
Quick steps
- Enter your markup in the HTML tab, styles in the CSS tab, and scripts in the JavaScript tab.
- Leave Auto-update preview on change enabled, or turn it off and use the Refresh button manually.
- Choose a viewport preset: Mobile, Tablet, or Desktop.
- Watch the live preview render inside the sandboxed iframe, automatically scaled to fit your screen.
- Click Copy to grab the combined HTML document, or Clear to reset all three editors.
- Real-time HTML preview with instant updates
- Separate editors for HTML, CSS, and JavaScript
- Auto-update preview on code changes (toggleable)
- Manual refresh button for control
- Safe sandboxed execution in isolated iframe
- Error handling for JavaScript errors
- Copy combined HTML output to clipboard
- Clear/reset all editors with one click
Data input / output
| Inputs | HTML, CSS, and JavaScript code entered in three separate tabs |
|---|---|
| Outputs | A live-rendered preview in a sandboxed iframe, viewable at three responsive viewport presets, plus a copyable combined HTML document |
| Formats | Standard HTML5 document assembled from your three code panels |
| Limits | The preview iframe uses sandbox="allow-scripts allow-same-origin", so scripts requiring top-level navigation or certain cross-origin behaviors may be restricted |
| Processing | Client-side (browser, sandboxed iframe) |
How to Use the Live HTML Previewer
- Enter your markup in the HTML tab, styles in the CSS tab, and scripts in the JavaScript tab.
- Leave Auto-update preview on change enabled, or turn it off and use the Refresh button manually.
- Choose a viewport preset: Mobile, Tablet, or Desktop.
- Watch the live preview render inside the sandboxed iframe, automatically scaled to fit your screen.
- Click Copy to grab the combined HTML document, or Clear to reset all three editors.
What is a Live HTML Previewer?
This previewer combines your HTML, CSS, and JavaScript from three separate tabbed editors into one HTML document and renders it inside a sandboxed iframe, updating automatically about 300 milliseconds after you stop typing (toggleable, with a manual Refresh button available when auto-update is off). JavaScript errors inside your code are caught and displayed as a visible red error message directly inside the preview rather than silently failing or crashing the whole page.
Three viewport presets — Mobile (375×667), Tablet (768×1024), and Desktop (1920×1080) — let you check how your layout responds at common screen sizes, with the preview automatically scaled down to fit your available screen space (never scaled up beyond 100%) so you can see the whole simulated viewport even on a smaller monitor. A Copy button grabs the fully combined HTML document (with your CSS in a style tag and JS in a script tag) for use elsewhere.
The tool assembles a complete HTML document string from your three code panels — CSS wrapped in a style tag, your HTML in the body, and your JavaScript wrapped in a try/catch block that displays any runtime error inline — then injects it into an iframe using the srcdoc attribute for safe, sandboxed rendering. This approach means your code runs isolated from the parent page while still executing real JavaScript and applying real CSS, giving an accurate preview of the combined result.
How it works
Live HTML Previewer reads your input from the panel above, applies real-time html preview with instant updates, and shows a result you can copy or download. Options stay visible so you can iterate without reloading the page.
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Common Use Cases & Examples
- Quick CSS layout testing: Paste a layout snippet and switch between viewport presets to check how it reflows across mobile, tablet, and desktop sizes.
- JavaScript snippet debugging: Write a small script and see runtime errors displayed directly inside the preview instead of only in the browser console.
- Sharing a quick prototype: Build a small interactive demo across the three tabs, then copy the combined HTML document to share or embed elsewhere.
Input / Output Examples
Input
Enter or paste the input the tool expects (text, numbers, or file).
Output
The result will appear in the output area and can be copied or downloaded.
Frequently Asked Questions
What is a live HTML previewer?+
A live HTML previewer is an online tool that allows you to write HTML, CSS, and JavaScript code and see the results instantly in a preview window. It provides real-time rendering without needing to save files or set up a web server.
How does the live HTML preview work?+
The live preview tool renders your HTML, CSS, and JavaScript code in a sandboxed iframe, updating automatically as you type (when auto-update is enabled). The code is isolated for safety and provides instant visual feedback.
Can I preview HTML without hosting it?+
Yes! Our live HTML previewer allows you to preview HTML, CSS, and JavaScript code without any hosting or server setup. Everything runs in your browser, making it perfect for testing and prototyping.
Is the HTML preview tool safe to use?+
Yes, the tool uses a sandboxed iframe with restricted permissions, preventing any malicious code from affecting your main browser. JavaScript errors are caught and displayed safely without crashing the preview.
Can I test HTML, CSS, and JavaScript together?+
Absolutely! The tool includes separate tabs for HTML, CSS, and JavaScript. All three are combined and rendered together in the preview, allowing you to test complete web pages with styling and interactivity.
Does the HTML editor have syntax highlighting?+
The editor provides a clean, monospace font for easy code reading. While not a full IDE, it's optimized for quick HTML, CSS, and JavaScript editing with proper formatting support.
Can I copy the previewed HTML code?+
Yes, you can copy the combined HTML output (including embedded CSS and JavaScript) using the copy button. This gives you the complete HTML document ready to use.
Is there a limit to how much code I can preview?+
While there's no hard limit, extremely large documents (1000+ lines) may take longer to render. The tool is optimized for typical web development tasks and prototyping.
Key Features
- Real-time HTML preview with instant updates
- Separate editors for HTML, CSS, and JavaScript
- Auto-update preview on code changes (toggleable)
- Manual refresh button for control
- Safe sandboxed execution in isolated iframe
- Error handling for JavaScript errors
- Copy combined HTML output to clipboard
- Clear/reset all editors with one click
- Mobile-responsive editor and preview layout
- No setup or hosting required
- Works entirely in your browser
Benefits
Prototype and test code without setting up a local development environment.
Catch JavaScript errors immediately with visible inline feedback.
Check responsive behavior across common device sizes in one place.
When to Use Live HTML Previewer
- Quickly testing a small HTML/CSS/JS snippet without setting up a local project
- Checking how a layout responds across mobile, tablet, and desktop viewport sizes
- Debugging a JavaScript error with immediate inline feedback in the preview
- Prototyping a UI idea before implementing it in a full project
Advantages
- Combined HTML, CSS, and JS editing with a single live preview
- Three responsive viewport presets with automatic scale-to-fit
- Sandboxed iframe rendering with inline JavaScript error display
- One-click copy of the fully combined HTML document
Tips
- Turn off Auto-update if you are making rapid edits and want to control exactly when the preview refreshes using the Refresh button.
- Use the Mobile viewport preset first when building responsive layouts, since it is often the most constrained case.
- Copy the combined HTML document when you want to paste the full working snippet into another editor or CodePen-style tool.
Common Mistakes
Expecting external JavaScript libraries to load automatically — you need to include a script tag or reference within your own HTML/JS if a library is required.
Assuming a script relying on browser features restricted by the sandbox attribute (like top-level navigation) will work identically to a non-sandboxed page.
Not noticing the auto-scale note at the bottom of the preview, which shows the current zoom percentage when the viewport is scaled down to fit your screen.
Summary
Write your HTML, CSS, and JavaScript above and watch the sandboxed preview update live across mobile, tablet, and desktop viewports — then copy the combined document once your snippet is ready.