It seems that I was trying to do everything too quick.
If I leave the adding of an event listener until frame 5, it works.
I think it was trying to add the event listener before (or at the same time as) it was trying to actually make the html5 video item.
There's probably a preload setting I should be using, but for now this workaround is doing the trick.
If somebody knows how to do this properly, I'd be grateful to hear how.
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How on Earth are you using HTML5 video in an AS3 document?
You were probably trying to address the component before it had initialized. Timeline code executes first, then any clips/components after that. Usually waiting one frame allows everything to fully initialize.