It does not. What you type in the three panes is used only inside this browser, and the preview is rendered by the browser itself. Even a share link carries the code after the # in the address, and browsers never send that part to a server. Your work is stored in this browser alone, so it will not appear on your other devices.

Every edit is saved in this browser automatically, so closing and reopening the page brings it back. That storage lives in the browser, though, so it goes away if you close a private window or clear your browsing data. For anything you want to keep, use Download to save a single HTML file, or make a share link and note it down — the link itself contains the code, so it opens the same way anywhere.

Add <script src="https://…"> in the HTML pane. Any CDN address works, and CSS libraries go in the same way with <link rel="stylesheet" href="…">. Those files are fetched from the internet, so that part alone will not work offline. Syntax that needs a build step first, such as JSX or TypeScript, does not run as-is — convert it to plain JS in the code converter and paste the result.

The preview runs in a sandbox with no origin of its own. That is what stops code you run here from touching this site's storage or its page — a fence worth having the moment you open a share link somebody else sent you. So that code does not die on its first line, a stand-in for localStorage and sessionStorage is installed instead. Its values live only for that run and are emptied when you run again. Cookies are not stored, for the same reason.

The preview reports that it is alive every 0.7 seconds. If that signal stops for more than four seconds, a banner appears and auto-run is switched off — otherwise every keystroke would freeze it again. Stop removes the preview. In some browsers the preview shares a process with this page, so the whole tab can stall; reload the tab in that case. Your code is saved automatically and comes back as it was.

Yes. The preview document puts the console bridge and your HTML in front of your script, but that offset is subtracted so the first line of the JS pane counts as line 1. Values logged with console.log are expanded too — objects, arrays, Maps and DOM elements — and circular references are cut rather than crashing. A single run shows at most 500 console lines, so a console.log inside a loop cannot take this page down with it.

You can. If the HTML pane begins with <!doctype html> or <html>, that document is used as the base: no second html, head or body is invented, and your <title> and lang are left alone. The CSS pane is appended at the end of head and the JS pane at the end of body. Paste a fragment instead and it goes inside body.

There is no sign-up, nothing is stored on a server, and once the page has loaded it keeps working with the internet disconnected (installing it as an app makes that more reliable). The trade-off is that without server storage there is no list of your saved work: you share by sending a link that carries the code. Very long code may not fit in a link, and then you can download the HTML file and send that instead.