That's enabled by default, on all major browsers, I'm fairly certain. Unless you click or have autoplayed in the past from that site it's a policy to not allow autoplaying video/audio unless the video is muted (<video> tag specifically)

As far as I'm aware I've never allowed anything to autoplay video. If a site has video on it then I immediately try to mute/stop it (hence my frustration with this site where there doesn't appear to be any way of stopping the video. I ended up closing the tab).

I'd suspect you clicked on something and they have `onclick=play()` or something.