Since, very stupid or very smart people decided to make FireFox look just like Chrome, FireFox is dying, and Chrome is slowly becoming as ubiquitous as Inernet Explorer was in 1999.

Unfortunately Google is an advertisement company, first. So, one often has to deal with shit such as autoplaying HTML5 videos on YouTube.

FireFox has a simple "about:config" flag that can kill HTML5 autoplay, but in Chrome, naturally, you kill things with more JavaScript.

Some of these just pause the video (not an elegant solution IMHO), others disable HTML5 altogether, so you fallback to the old, trustworthy and secure, click-to-play Adobe Flash.

Works on "youtube.com":

YouTube HTML5 AutoPause
https://greasyfork.org/en/scripts/4944-youtube-html5-autopause
Youtube Html5 Disable
https://greasyfork.org/en/scripts/5642-youtube-html5-disable
YouTube Stop HTML5 Video Autoplay 2 Simple
https://greasyfork.org/en/scripts/8982-youtube-stop-html5-video-autoplay-2-simple

Works on all websites:

Disable audio/video autoplay
https://greasyfork.org/en/scripts/8022-disable-audio-video-autoplay
HTML5 video auto-pause
https://greasyfork.org/en/scripts/388-html5-video-auto-pause
Pause all HTML5 videos on load
https://greasyfork.org/en/scripts/6487-pause-all-html5-videos-on-load

Also see:

"YouTube Links" (download videos as mp4s, etc.)
https://greasyfork.org/en/scripts/5566-youtube-links
and
http://insurgencymod.blogspot.com/2015/03/quotes-anti-web-brosign-manifesto.html

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