Problems with that: full screen the video and the poster disappears; if the video aspect ratio differs from the poster, or the video has an alpha channel, the p

Problems with that: full screen the video and the poster disappears; if the video aspect ratio differs from the poster, or the video has an alpha channel, the poster will be visible underneath; browsers could change their video poster styles so that this would look different or even not work (though at least in Firefox and Chromium they do look the same at present).