Recording
Choose what to add, press Start recording and the browser asks what to share: the whole screen, one window or one browser tab. After the countdown it records until you press Stop, or until you stop sharing from the browser's own bar. Pause leaves a gap out of the video rather than recording it. The clock and the size so far are shown as it goes, and the recording is written out in one-second pieces, so a long one does not pile up in memory all at once.
Sound, voice and the webcam bubble
Sound from the screen takes a tab's sound in Chrome and Edge (tick Share tab audio in the picker), or the whole computer's sound when sharing the entire screen on Windows and ChromeOS; macOS and Firefox do not share system sound with web pages. Microphone mixes your voice in with it. Webcam bubble puts your camera in a corner, round or with rounded corners, in three sizes; it is drawn into the video itself, so it is there in every player.
The file
Where the browser can record MP4 (H.264 video and AAC sound), the recording is an MP4 that plays everywhere. Otherwise it is WebM (VP9 or VP8 with Opus), which plays in Chrome, Firefox, Edge and VLC; the line under the Start button says which before you begin. Play it back on the page, then download it. To cut the ends off or make an MP4 from a WebM, open it in Trim Video.
Questions
Is there a time limit or a watermark?
No. It records until you stop it, limited only by the space your browser may use, and nothing is added to the picture. Nothing is uploaded either: the video is made in this page and saved straight to your device.
Can I record my screen on a phone?
Mostly not. Phone browsers, on iPhone and Android alike, do not let web pages capture the screen. Use the phone's own screen recorder for that. Here you can still record from the camera, with or without the microphone.
Why is there no sound from my computer?
Browsers only share a tab's sound, or the whole system's on Windows and ChromeOS, and only if you tick the audio box in the share picker. On a Mac, share a browser tab with its audio, or play the sound out loud and record it with the microphone.
Why is my recording a WebM?
Your browser cannot record MP4 itself. WebM plays in most browsers and in VLC; for a phone or an editor that wants MP4, open the file in the Video Editor and save it as MP4.