You will need to install a user script manager extension to install this script.
Prevents auto-play HTML5 videos in new tabs on any page (not just YouTube) and pauses videos when leave/switch/change the current tab, to prevent any video playing in the background.
This auto play blocker is an alternative to Click-to-Play / click to play. You can edit user settings variables near the top of the script for how long after a click occurs to allow starting playback of video (default is .5 seconds).
NOTE: On YouTube, currently need to click twice to begin playing the first time.
Handles YouTube and all other video sites/pages. Optimized for highest-performance compared to other scripts and avoids many cases of breaking video controls or seeking.
NOTE: On YouTube, click the main video area to play the video so don't have to click the Pause/Play button twice for the first time.
By Dan Moorehead