MediaWiki:Common.js

$(document).ready(function() {
  if (!$('#bg-video-wrapper').length) {
    $('body').prepend(`
      <div id="bg-video-wrapper">
        <video autoplay muted loop playsinline id="bg-video">
          <source src="/resources/assets/bg.webm" type="video/webm">
        </video>
      </div>
    `);
  }
});