YoutubeHTML5StopAutoplay.user.js

// ==UserScript==
// @name           Youtube HTML5 stop autoplay
// @namespace      http://smiler.se/
// @description    Prevent HTML5 videos on youtube from autoplaying
// @include http://www.youtube.com/*
// @include https://www.youtube.com/*
// @exclude http://www.youtube.com/embed/*
// @exclude https://www.youtube.com/embed/*
// @match http://www.youtube.com/*
// @match https://www.youtube.com/*
// @match http://s.ytimg.com/yts/jsbin/html5player*
// @match https://s.ytimg.com/yts/jsbin/html5player*
// @match http://*.googlevideo.com/videoplayback*
// @match https://*.googlevideo.com/videoplayback*
// @match http://*.youtube.com/videoplayback*
// @match https://*.youtube.com/videoplayback*
// @version       1.0
// ==/UserScript==

function onLoad() {
	setTimeout(function(){
    	video = document.getElementsByTagName('video')[0];
		if (video) {
		    video.pause();
		    video.currentTime = 0;
		}
	}, 300);
}

window.addEventListener('load', onLoad, false);