https://www.theverge.com/2017/9/25/16352736/macos-high-sierra-review-safari-autoplay-blocking

Apple's new version of macOS comes out today, and while almost every upgrade is under the hood and out of sight, there is one really great new feature inside Safari that is definitely worth updating for: in High Sierra, Safari automatically mutes autoplaying videos. The update to High Sierra is free and will be available for all Macs released since 2010, and some Macs introduced in 2009 as well.

I've been browsing the web in Safari for the past couple weeks, and the internet has felt like a somewhat calmer place, thanks to this new feature. If a video would normally autoplay with sound, it'll still appear, but Safari will automatically pause it before the video can start to emit noise. That way, you get to browse in peace and quiet but can still hit play in the (unlikely) event that the autoplaying video was something you were interested in.