/* Splash video */
    #splashVideo {
      position: fixed;
      top: 0; left: 0;
      width: 100%; height: 100%;
      background: #000;
      display: none;
      z-index: 1500;
    }
    #splashVideo video {
      width: 100%; height: 100%;
      object-fit: cover;
    }

    /* Main content */
    #mainContent {
      display: none
      
<!-- Splash Video -->
  <div id="splashVideo">
    <!-- autoplay + muted ensures it plays automatically -->
    <video id="video" src="splash.mp4" autoplay muted></video>
  </div>