Is my code sent to a server?
No. The HTML/CSS/JS runs entirely in your browser; no line is ever sent to any server.
Does my code run isolated?
Yes. The preview runs in a separate sandboxed iframe without allow-same-origin, so your code cannot reach the site's cookies, localStorage or DOM.
Will I lose the code I write?
No. Your code is saved automatically in your browser, so you pick up where you left off even after a reload. For a permanent copy, the "Download" button gives you all three parts as a single index.html file.
What happens if I write an infinite loop?
If the preview doesn't respond within 3 seconds, the tool automatically recreates the preview iframe and shows a "code is not responding" warning; the page itself does not freeze.