Indeed setting autoplay to false doesn't help some videos will play anyway. See this case in fiddle.
You might want to do by code something in the line of if you want to pause all the videos:
videos = document.querySelectorAll("video");
for(video of videos) {
video.pause();
}
Of course the above case will not work if the video tag is in a shadow root element, but then hardly any general solution will work with shadow roots elements. There you will need a custom approach and expand first the shadow roots.