Why is JavaScript off by default?

Most of the time you're previewing markup, not testing a script — and defaulting to off means you never have to think about it unless you specifically want to test interactive behavior. Turning it on is one click away when you do.

Can the preview navigate my tab, open a popup, or show an alert?

No. The sandbox blocks top-level navigation, popups, and dialog boxes (alert/confirm/prompt) regardless of whether JavaScript is enabled.

Is this the same as the Markdown to HTML preview?

No — that preview sanitizes the rendered HTML (stripping anything script-like) before showing it directly on the page, which works because Markdown output is inherently limited. This tool is built for arbitrary HTML, including scripts if you want them, so it uses the stronger isolation of a sandboxed iframe instead.

HTML Viewer Online

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Preview raw HTML as a rendered webpage — in a sandboxed iframe with no access to this site's cookies or storage.

✓ Never uploaded — rendered in your browser

The preview is sandboxed — it cannot read this page's cookies or storage, cannot reach outside the preview frame, and cannot navigate your tab or open popups, even with JavaScript turned on. Only paste HTML from sources you trust.

HTML source

Sandboxed preview

Your rendered HTML will appear here

Never uploaded

Rendering happens entirely in your browser — your HTML never touches our servers.

Isolated by design

A sandboxed iframe with no shared cookies, storage, or DOM access — not even when JavaScript is enabled.

Free, no limit

No account, and no limit on how many times you can use it.

How the Sandbox Works

The preview renders inside an <iframe> with the browser's built-in sandbox attribute — a real security boundary the browser itself enforces, not something this tool has to implement by hand. With JavaScript off, <script> tags and inline event handlers simply don't run at all. With JavaScript on, scripts do run, but the sandbox deliberately omits the one permission (allow-same-origin) that would let them access this page's cookies, local storage, or DOM — and top-level navigation, popups, and dialog boxes stay blocked in both cases.

How to Preview HTML Online

Paste or upload your HTML. Paste directly into the editor, or click "Upload a file" to load an .html file's contents.
Read the preview. It updates automatically as you type.
Turn on JavaScript (optional). Off by default — switch it on only if you want to test interactive behavior.
Download (optional). Save your HTML as a local .html file.

Frequently Asked Questions

Is it safe to preview HTML I don't fully trust?

The preview runs in a sandboxed iframe with no access to this site's cookies or storage. Even so, only paste HTML from sources you trust.

Does JavaScript run in the preview?

Only if you turn on "Run JavaScript," which is off by default.

Is my HTML uploaded anywhere?

No. Rendering happens entirely in your browser.

Is the HTML viewer free?

Yes, with no account required and no limit on how many times you can use it.

Preview your HTML now