var videoElements = document.getElementsByTagName("video"); for (var videoElement of videoElements) { videoElement.removeAttribute("autoplay"); } ..

var videoElements = document.getElementsByTagName("video"); for (var videoElement of videoElements) { videoElement.removeAttribute("autoplay"); } ...in a Userscript ought to do it. HTML5 gives more power to those who can write code than we had with Flash. That said, this does not qualify as easy for most users. If there is enough demand for a "never autoplay" feature among users, browers ought to implement an option in the user interface. It is not a good reason for keeping Flash around.