<div id="dom_overlay_container" style="z-index:-1; pointer-events:none; overflow:hidden; width:160px; height:600px; position: absolute; left: 0px; top: 0px; display: block;">
and set transparent background to canvas
Why did you put your CSS code inside braces?
I added z-index and set the movie clip symbol to Render: Cache as Bitmap / Transparent. Here is the code I'm using:
makeDiv(980, 210, 640, 480, "<video src='lilli.mp4' autoplay ></video>", style="z-index:-1;");
The video loads, but there is a white where the video should be.
I double checked my image to make sure its transparent. I also moved the symbol to a second layer and wrote something on a bottom layer and there is transparency.