Quick Summary
HTML Online Viewer is a real-time DOM sandbox that renders HTML5, CSS, and inline JavaScript inside an isolated browser iframe, offering Desktop (100%), Tablet (768px), and Mobile (375px) responsive viewport testing.
HTML Live Preview & Device Viewport Sandbox
Test and preview HTML markup live inside an isolated browser sandbox with Desktop, Tablet, and Mobile device frame viewports.
Device Viewport Presets
Switch between Desktop (100%), Tablet (768px), and Mobile (375px) viewports with one click.
New Tab Isolated Rendering
Launch your rendered HTML page into a dedicated browser tab window with Alt+2.
HTML Code Beautifier
Automatically re-indent nested DOM elements and tag hierarchies for clean source code.
Frequently Asked Questions
How do I test responsive viewports in the HTML preview?
Click the Desktop, Tablet, or Mobile viewport buttons in the toolbar to change the sandbox iframe container width instantly.
Is external JavaScript supported in the sandbox?
Yes. Inline scripts and CDN script tags execute inside the isolated sandboxed iframe securely without affecting the main application window.