To preview HTML and test responsive layouts: Type or paste HTML markup into the HTML Sandbox Tool. The DOM updates instantly inside a sandboxed iframe srcdoc element. Toggle between Desktop (100%), Tablet (768px), and Mobile (375px) viewports to verify media queries without leaving your browser.
How HTML Live Preview Sandboxes Work
Modern frontend web development requires fast feedback loops when editing HTML templates, CSS styles, and UI components. Rather than saving files and reloading local servers, OnlineViewer Dev uses an HTML5 iframe srcdoc container to render DOM nodes dynamically in real-time as you type.
Key Viewport Testing Presets
- Desktop Viewport (100%): Full-width container rendering for large monitor resolutions.
- Tablet Viewport (768px): Simulates iPad and tablet screens to verify medium breakpoint media queries.
- Mobile Viewport (375px): Simulates iPhone and mobile screens to test flexbox wrapping, touch targets, and mobile navigation bars.
Security: Why Sandboxing Protects Your Browser Session
Executing untrusted HTML or third-party code snippets in the main document object model (DOM) can risk cross-site scripting (XSS) or DOM poisoning. OnlineViewer Dev isolates all HTML previews with explicit sandboxing policies:
Launching Previews in New Isolated Tabs
If you need to test full window resizing or popup triggers, click New Tab Preview (or press Alt+2) to launch your rendered HTML document into a dedicated browser tab window instantly.