I bookmarked a video at YT recently, right in the middle of it, and today when I opened it again it resumed where I'd stopped last time! Is this a feature of the site, or of my browser (FireFox)? Whatever it is I'm pretty sure it wasn't there before, and I can't find any information about it.
In fact, when I Google for results on 'YouTube automatically resumes video' I get links to all types of plugins with which you can achieve this result. But it seems you don't need plugins any longer. Whatever feature and wherever it´s been added, I love it! It's all so easy now.