Note that the video won’t play on a mobile device. In order to have the poster image display nice on your mobile device, I would suggest you add to the css the following (I tested this specifically on my iPhone so you might want to take this a step further and test on other devices including tablets):
@media( max-width: 480px ) {
video[poster] {
object-fit: cover;
width: 100%;
height: 100%;
}
}
Otherwise, your poster image may simply be zoomed in on an irrelevant portion of the image.