Is this HTML editor safe to run JavaScript in?

Yes. The preview runs in a sandboxed iframe with allow-scripts and allow-forms permissions. The code cannot access parent page state, cookies, or localStorage — it is fully isolated from the rest of the page.

Can I use CSS and JavaScript in the HTML editor?

Yes. You can write a complete HTML document with embedded and blocks. Inline CSS, external-like style rules, and JavaScript all render in the preview iframe.

Can I download the HTML I write?

Yes. Click the Download button to save the current editor content as index.html to your local machine.

Does the HTML viewer auto-update as I type?

Yes. Auto mode updates the preview 600ms after you stop typing so the page does not re-render on every keystroke. You can switch to Manual mode and click Run to control when the preview updates.

All tools

🖥️

HTML Viewer & Live Editor

Write HTML, CSS and JavaScript in separate tabs — live preview, built-in console, resizable panes

100% in-browserHTML · CSS · JS tabsBuilt-in consoleZoom & viewportShare via URL

100%

151 chars

Live

HTML Viewer — Preview HTML, CSS and JavaScript Instantly

The HTML Viewer gives you a full live code editor and sandboxed preview side by side. Write or paste any HTML — with embedded <style> and <script> blocks — and see it render in real time without leaving the browser. The preview runs in an isolated iframe so JavaScript executes safely without affecting the rest of the page.

Use the five built-in starter templates (blank page, card UI, login form, gradient hero, CSS animations) to jump-start your code. Switch between desktop and mobile viewport widths, toggle a dark preview background, and copy or download the finished HTML with one click. Everything runs 100% client-side — no server, no signup.

How it works

Live HTML Preview in 10 Seconds

Choose a template

Pick from Blank, Card UI, Login Form, Gradient Hero, or CSS Animation — or paste your own HTML.

Edit the code

Write HTML, CSS, and JavaScript in the code panel. Auto mode updates the preview as you type.

Preview instantly

See the rendered output in the live preview panel. Switch between desktop and mobile viewports.

Copy or download

Copy the HTML to clipboard or download as index.html for immediate use in your project.

Use cases

When to Use an Online HTML Viewer

🧪

Prototype UI components

Quickly test CSS layouts, animations, and component ideas without setting up a local project.

📧

Preview email HTML

Paste HTML email templates and check how they render before sending to a client or test inbox.

🎓

Learn HTML and CSS

Students and beginners can experiment with HTML tags and CSS rules and see results instantly.

🔍

Debug rendered markup

Paste server-generated HTML to check structure, class names, and styles in isolation.

🎨

Test CSS animations

Build and fine-tune CSS keyframe animations with a live preview that updates as you type.

📋

Share HTML snippets

Write a self-contained HTML snippet for documentation, a blog post, or a code review.

FAQ

Frequently Asked Questions

1Can I use external CSS libraries like Tailwind or Bootstrap?

Yes — add a <link> tag pointing to a CDN URL inside your <head>. For example, add the Tailwind CDN script tag and Tailwind utility classes will work in the preview. The sandbox allows network requests for loading CDN resources.

2What is the sandbox attribute used in the preview iframe?

The preview iframe uses sandbox="allow-scripts allow-forms allow-modals". This means JavaScript runs, forms can be submitted, and alert/confirm dialogs work — but the code cannot access parent page cookies, localStorage, or navigate the outer page.

3Is there a file size limit?

No hard limit — the editor is a standard textarea. Very large HTML documents (100KB+) may slow the preview slightly since the browser needs to parse and render them on each update. Use Manual mode for large documents to control when re-rendering happens.

4Can I use multiple files (CSS file + JS file)?

This is a single-file sandbox. Put all CSS in a <style> block and all JavaScript in a <script> block inside the same HTML document. For multi-file projects, use a local code editor like VS Code.

Feedback for html_viewer

Tell us what's working, what's broken, or what you wish we built next — it directly shapes our roadmap.

You make the difference

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Feature ideas, bugs, or a quick thanks — we read every message.

support@unblockdevs.com