I have tried and tried but still no joy.....I have put the video file in the index_htm_files
and now I have put this code into the head of the placeholder

<style>#myVideo { position: fixed; left: 0; top: 0; min-width: 100%; height: auto;}</style><video autoplay muted playsinline loop id="myVideo"><source src="https://index_htm_files/testvideo.mp4" type="video/mp4"></video>

where have I gone wrong .....lol.
I copied the code from your example page but change the source.

Thanks for any help
Mike