Media Capture and Streams API (Media Stream) - Web APIs | MDN[
^]
And before you ask: NO, you cannot automatically start recording the user as soon as they open your site. You have to get their permission first.
Also, before you come back with
"I've already tried that", or
"It doesn't work": read your question again. Then explain precisely where in your question you actually tell us
what you have tried, or
why it didn't work.