HTML Runner

Edit the markup on the left, then press Run.

Semantic markup test

• A link
• Bold and italic

What Is an Online HTML Runner?

An online HTML runner is a browser-based editor that renders your markup the moment you press Run. It removes every step normally required to see a page: no folder to create, no editor to install, no local server to start, no build tool to configure. You write HTML on one side and watch the browser paint it on the other.

This runner is built for people who need a fast, trustworthy answer to a simple question: does this markup do what I think it does? That question comes up dozens of times a day for developers, students, marketers pasting blocks into a CMS, and anyone learning how the web is structured.

How the Sandbox Keeps You Safe

Executing arbitrary code in a browser tab is only safe when that code is isolated. The preview here is an iframe created with a sandbox attribute that grants scripts, modals, popups and form submission — but deliberately withholds allow-same-origin. Without that permission the frame receives an opaque origin, meaning the code inside it is treated as belonging to a different site than Zubac Tools.

The practical result: markup you paste cannot read the page around it, cannot touch cookies, cannot read or write this site's localStorage, and cannot call privileged browser APIs on our behalf. Console output and runtime errors are forwarded across the boundary using postMessage, and every message is checked to confirm it came from the preview frame before it is displayed.

Reading Errors Properly

Silent failure is the most frustrating part of debugging markup. This runner captures console.log, info, warn, error and debug, uncaught exceptions via window.onerror, and unhandled promise rejections. Errors arrive with the line and column reported by the browser, and objects are serialised so you can read their shape instead of seeing [object Object].

Editor Features That Matter

• Syntax highlighting and folding for real HTML, powered by a full code editor rather than a textarea.
• Line numbers and bracket matching so error messages map to what you see.
• One-click formatting that reindents the entire document consistently.
• Import and download to move between this playground and your local files.
• Fullscreen mode and resizable panels for working comfortably on large layouts.
• Ctrl/⌘ + Enter to run without reaching for the mouse.
• Automatic local drafts so a refresh never costs you work.
• Light and dark themes that follow the rest of the site.

Common Ways People Use It

Checking whether a snippet copied from documentation actually works. Teaching a beginner what a tag does by changing it and re-running. Verifying that a marketing block renders correctly before pasting it into a landing page builder. Reproducing a rendering bug in isolation so it can be reported clearly. Testing accessibility basics such as heading order, alt text and label association. Building a tiny reference page for a component while designing it.

HTML Runner vs Local Development

A local project wins for anything with dependencies, routing, a bundler or a real backend. A browser runner wins for everything smaller: an experiment, a lesson, a snippet review, a quick reproduction. Because there is no install step, the runner is also the only option on a locked-down work machine, a school computer, a tablet or a phone.

Privacy

All processing happens locally in your browser. No markup, console output or file is transmitted to Zubac Tools or any third party. Drafts are stored only in your own browser and can be cleared at any time with the Reset button.

Continue with the HTML, CSS & JavaScript Runner for multi-file projects, tidy your output with the HTML Minifier, or check document structure with the Heading Structure Checker.

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