In recent days, the way that CNN autoplays its videos has changed and old methods of blocking them no longer work. I suspect it has something to do with their using HTML5 instead of Flash. Any suggestion on how to block them now?
After further searching, I've found plug-ins for both Firefox and Chrome that handle both the Flash and HTML5 attempts to auto-play videos on CNN. It appears that CNN is now trying up to six times to auto-play a video, and these plug-ins catch them all! https://www.technologyhint.com/stop-video-autoplay-chrome-firefox/
You can try this extension in Chrome:
Disable HTML5 Autoplay