Is there a way in HTML to have elements overlap "on purpose" ??
I have a slideshow I'd like to cover up the control bar as it is inactive and thus just taking up space.
I'm just going to say up-front that this is a world of pain that I would not recommend.
Getting pixel-perfect alignment across the wide variety of browsers, platforms, screen resolutions and devices is just a nightmarish combinatorial problem that you probably don't want to deal with. It can probably be done, but be prepared for the pain of managing a variety of little workarounds for the edge cases, which you will probably only discover because someone complains that things have gone horribly wrong for them (your overlay is in the middle of the video, etc, etc...).
From a pragmatic perspective, you're probably better off finding a video control that gives you the design options you're looking for, vs. trying to hack this into submission. There are plenty of good (commercial, reasonably priced) options in the Flash-based video player world. I'm thinking that the HTML5 video tag also sports some options (if you're not using Flash Player for video playback), but I'm less familiar with them.