Recording without installing anything
Every desktop browser has had a screen-capture API for years — the same one video-call software uses when you share your screen. This page asks for it, hands the resulting stream to the browser's built-in recorder, and gives you the file. There is no server in the path, which is why there is no upload wait, no watermark and no account.
The audio question
Capturing sound is the part that confuses everyone, because the control belongs to the browser rather than to this page. When you press record, the browser shows its own picker with tabs, windows and whole screens — and a share-audio checkbox that only appears for tabs and screens, never for a single window. If you need the sound from an application, share the whole screen.
On macOS and in Firefox, system audio capture is not implemented at all. Recording your microphone works everywhere, so narration is always available even when the system sound is not.
Why the format depends on your browser
The recording is produced by the browser's own encoder, and each browser ships a different one. Chrome, Edge and Firefox write WebM with VP9 or VP8; Safari writes MP4. Chrome deliberately does not expose an MP4 encoder to web pages, so no site can offer you MP4 there — anything claiming to is converting afterwards, which you can do here too with the video converter.
Download before you close the tab
Because nothing is uploaded, the recording exists only in the memory of this page. Closing or reloading the tab discards it, and there is no server-side copy to recover. Download first.