This is the same on all handheld and mobile devices. Autoplaying media is disabled by the device because if media started autoplaying when you went to a website your monthly data download limit would soon be exceeded. As for doing this in Web Designer, no you cannot add a placeholder to the background. If you really wanted to do this then adding a placeholder to the MouseOff layer the size of the page background would be my answer off the top of my head without trying it. You should still be able to place objects on top of it like they have.
Wasn't there a thread a the other day in Off-Topic on how to disable autoplay in browsers....?
For me, sites with autoplaying audio and video are in the same league as animated GIF backgrounds as far as how quickly it takes me to find another website
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