I managed to show a local background video with a header element.
I do not understand this part, however:
<div class="background-video-content">
Your Element content in here.
</div>
Should the text in there be an overlay over the video or can I make another element and that gets combined with the video? With using as it is, the text just shows up under the video.
Then in the Header Element add some Top and Bottom Padding to set the image size. Use % padding to keep it responsive. Try 28% top and 28% bottom padding – this will be roughly the same aspect ratio as the video.