A service worker installs into the browser the first time you visit a site. After that, it can intercept every network request from that site (or specific patterns), serving from cache if available, fetching from network if not. This is what makes YouTube's navigation feel instant — recently viewed pages are cached.
The downside: a corrupted service worker can cache broken state. A YouTube page that's "stuck on loading" sometimes has a service worker returning a stale, broken version. Clearing site data in DevTools (Application → Storage → Clear site data) removes the service worker, which fixes it.
For VidPickr: we don't register a service worker because the architecture is intentionally simple (server-side rendering + client-side hydration). The StreamSaver fallback library does register one as part of its offline-save trick, but it's scoped to mitm subdomain and doesn't affect normal navigation.
Common questions
What does "clear site data" actually do?
Related terms
Cookie (browser session)
A cookie is a small piece of data a website stores in your browser to track session state — whether you're logged in, which preferences you've set, what country you appear to be in.
CORS (Cross-Origin Resource Sharing)
CORS (Cross-Origin Resource Sharing) is the browser security rule that prevents JavaScript on one domain from making requests to a different domain unless the target explicitly allows it.
VidPickr is a free, browser-based YouTube downloader. Every term in this glossary either describes how YouTube delivers video or why your downloads behave the way they do. Try the downloader →