One option was to go down the Click-to-Play route which offers a HORRIBLE UX. Especially on Youtube which still uses Flash by default. Disabling Flash however, Youtube actually seamlessly falls back to HTML video. Well done. But I can't help but think, outside the Youtube world (BBC for e.g.). LOTS is going to break. I wouldn't take this tact with my parents or clients.
My browsing workflow involves a lot of middle-clicking links to open them up in background tabs. Autoplaying video is major annoyance in this context.