Yes, this is very strange. Sometimes in my testing the poster images (poster
attribute to the video element) do get downloaded and other times not. Also
sometimes the src attribute of the source elements within the video element
get downloaded and sometimes not.

If there's no current support for these, is there some other mechanism which
allow them to be downloaded sometimes but not others? I'm just trying to
understand the issue so that I can work around them in the meantime.