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Responsive Video in Section

I want to achieve something like this:

Where the video scales in its element, which fits the browser viewport. I can understand most of what is going on, except it seems the div with the background video (it is not a WF background video element I believe… or is it?) - has its height adjusted dynamically in its element style setting… can anyone enlighten me further and can this be done without using the WF background video element which I understand compresses quite heavily and has filesize restriction…

many thanks