Unfortunately, its way too easy to set a web video to auto-play, and effectively lock the player controls preventing a user from stopping it manually.
For example, click the above video to watch this
funny commercial(3.5MB). The top video is set to auto-play. Try stopping the video? Disabling Javascript or Flash has no effect, as it's HTML5 embedded video?
In contrast, the bottom video is the same commercial, but without auto-start and with standard browser playback controls. What's the difference?
The top video was placed in its own "div" set to "z-index:-1;", so its on a layer below the other video. But since nothing is blocking it, you can watch the top video, just don't have access to the player controls. Like walking on a glass floor which allows you to see the level below you.