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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/captureStream below:

HTMLMediaElement: captureStream() method - Web APIs

HTMLMediaElement: captureStream() method

Limited availability

The captureStream() method of the HTMLMediaElement interface returns a MediaStream object which is streaming a real-time capture of the content being rendered in the media element.

This can be used, for example, as a source for a WebRTC RTCPeerConnection.

Syntax Parameters

None.

Return value

A MediaStream object which can be used as a source for audio and/or video data by other media processing code, or as a source for WebRTC.

Examples

In this example, an event handler is established so that clicking a button starts capturing the contents of a media element with the ID "playback" into a MediaStream. The stream can then be used for other purposesâlike a source for streaming over WebRTC, to allow sharing prerecorded videos with another person during a video call.

document.querySelector(".playAndRecord").addEventListener("click", () => {
  const playbackElement = document.getElementById("playback");
  const captureStream = playbackElement.captureStream();
  playbackElement.play();
});

See Recording a media element for a longer and more intricate example and explanation.

Specifications Browser compatibility Firefox-specific notes

Prior to Firefox 51, you couldn't use captureStream() on a media element whose source is, itself, a MediaStream (like a <video> element which is presenting a stream being received over a RTCPeerConnection). Beginning in Firefox 51, this works. This means you can capture a stream from the video element and use MediaRecorder to record it. See Firefox bug 1259788 for details.

However, captureStream() is still prefixed as mozCaptureStream() on Firefox for good reason: there are some quirks in the present implementation which are worth noting:

The Firefox implementation currently only works as described in the specification when the media element's source is, itself, a MediaStream.
If the media element's source isn't a MediaStream, the output from this method isn't compatible with the spec, and if you change the source after starting capture, the output capture stream can't accept the new source data due to that incompatibility, so no MediaStreamTracks from the new source MediaStream are added to the captured stream, resulting in output that doesn't capture the updated source.
Switching the source back to a MediaStream adds tracks back to the stream and it works again as expected.
Calling mozCaptureMediaStream() on an element with a MediaStream source returns a stream that only contains tracks while the element is playing a MediaStream.
If you call mozCaptureMediaStream() on a media element with no source media, its compatibility mode will be based on the first source that's added; for example, if it's a MediaStream, then the capture stream will only work with MediaStream sources from then on.
This special behavior will be removed once the non-MediaStream source support is brought up to specification and the method is unprefixed. See Firefox bug 1259788 for details.