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CSS

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JavaScript

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HTML 

              
                <a href="https://youtu.be/lA4sqcZhPx4" target="_blank" data-keyframers-credit style="color: #000"></a>
<script src="https://codepen.io/shshaw/pen/QmZYMG.js"></script>
<div id="app">
  
  <header class="ui-header">
    <div class="ui-subheading">The best show you've ever seen</div>
    <h1 class="ui-heading">@keyframers</h1>
  </header>
  
  <div class="ui-arrow"></div>
  
  <nav class="ui-nav">
    <a href="#">Coding</a>
    <a href="#">Animating</a>
    <a href="#">Streaming</a>
    <a href="#">Punning</a>
  </nav>
  
  
  <div class="video-tile -first">
    <img src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/39255/kf_glacier_1.jpg" />
    <video src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/39255/kf_glacier_1.mp4" preload="auto" loop="true"></video>
  </div>
  
  <div class="video-tile -second">
    <img src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/39255/kf_glacier_2.jpg" />
    <video src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/39255/kf_glacier_2.mp4" preload="auto" loop="true"></video>
  </div>
  
  <div class="video-toggle">
    <div class="video-toggle-inner">
      <video src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/39255/kf_glacier_3.mp4"></video>
    </div>
  </div>
</div>
!

CSS 

              
                @import url("https://fonts.googleapis.com/css?family=Open+Sans:400,700");

$color-primary: #4066c8;

html {
  font-family: "Source Sans Pro", sans-serif;
}

html,
body {
  height: 100%;
  width: 100%;
  margin: 0;
  padding: 0;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  background: #ddd;
}

* {
  position: relative;
  box-sizing: border-box;
}

body,
#app {
  &:before {
    content: "";
    display: block;
    position: absolute;
    top: 0;
    bottom: 0;
    left: 50%;
    width: 35vw;
    background: $color-primary;
  }
}

body:before {
  background: darken($color-primary, 8%);
}
// ---------------------------
#app {
  background: #fff;
  width: 90vw;
  height: 70vh;
  overflow: hidden;
  box-shadow: 0 0.5rem 3rem rgba(black, 0.1);
  padding: 2rem 3rem;
  
  display: grid;
  grid-template-rows: 1fr 1fr 1fr;
  grid-template-columns: 1fr;

  // 3d environment
  transform-style: preserve-3d;
  perspective: 1000px;
}

.ui-header {
  grid-row: 1 / 2;
  align-self: end;
}

.ui-heading {
  margin: 0;
  font-weight: 300;
  font-size: 2.5rem;
}

.ui-subheading {
  text-transform: uppercase;
  font-size: .6rem;
  color: #aaa;
  font-weight: bold;
  letter-spacing: .5px;
  margin-bottom: .1rem;
}

.ui-heading,
.ui-subheading,
.ui-nav > a {
  animation: slide-left 2.5s cubic-bezier(0, 1, .2, 1) both,
    fade-in 0.6s linear both;
}

@for $i from 1 through 4 {
  .ui-nav > a:nth-child(#{4 - $i}) {
    animation-delay: .2s * $i, .2s * $i;
  }
}

@keyframes fade-in {
  from { opacity: 0; }
}

@keyframes slide-left {
  from { transform: translateX(-2rem); }
}

.ui-nav > a {
  text-decoration: none;
  color: #444;
  font-weight: 600;

  ~ a {
    opacity: 0.3;
  }
}

.video-tile,
.video-toggle {
  position: absolute;
}

.video-tile {
  // display: none;
  position: absolute;
  overflow: hidden;
  z-index: 1;
  transition: transform 0.4s linear;

  video {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    max-width: none;
    max-height: 100%;
    min-width: 100%;
    min-height: 100%;
    z-index: -1;
  }
}

.video-tile.-first {
  left: 30%;
  bottom: 10%;
  width: 50%;
  animation: video-tile-enter 2.5s cubic-bezier(0, 1, .2, 1) backwards;
}

.video-tile.-second {
  right: 5%;
  top: 10%;
  width: 30%;
  animation: video-tile-enter 3s cubic-bezier(0, 1, .1, 1) backwards;
}

@keyframes video-tile-enter {
  from {
    opacity: 0;
    pointer-events: none;
    transform: translateX(30%) rotateY(20deg);
  }
  to {
    opacity: 1;
    transform: none;
  }
}

.video-tile img {
  width: 100%;
  opacity: 1;
  transition: opacity 0.3s linear;
  display: block;
}

.video-tile:hover,
.video-tile:focus {
  z-index: 2;
  // transform: scale(1.1) !important;
  img {
    opacity: 0;
  }
}

.video-toggle {
  overflow: hidden;
  transform: translate(calc(50% - 5rem), calc(50% - 5rem));
  z-index: 9;
  visibility: hidden;
  cursor: pointer; 

  &,
  .video-toggle-inner {
    position: absolute;
    width: 100%;
    height: 100%;
    overflow: hidden;
    transition: transform 0.6s cubic-bezier(0.6, 0, 0.4, 1);
  }

  .video-toggle-inner {
    transform: translate(calc(-100% + 10rem), calc(-100% + 10rem));

    &:before {
      pointer-events: auto;
      content: "▶";
      height: 10rem;
      width: 10rem;
      border: solid 1px white;
      bottom: 0;
      right: 0;
      position: absolute;
      z-index: 1;
      display: flex;
      justify-content: center;
      align-items: center;
      font-size: 5rem;
      background-color: white;
      color: $color-primary;
      text-shadow: 0 0.25rem 1rem rgba(black, 0.1);
      transition: opacity 0.3s linear 0.3s;
      visibility: visible;
    }
  }

  video {
    transform: translate(calc(-50% + 15rem), calc(-50% + 10rem));
    position: absolute;
    top: 50%;
    left: 50%;
    min-width: 100%;
    min-height: 100%;
    max-width: initial;
    width: auto;
    transition: inherit;
    visibility: visible;
  }

  &.active {
    pointer-events: auto;

    &,
    .video-toggle-inner {
      transform: none;
    }

    .video-toggle-inner:before {
      opacity: 0; 
      transition-delay: 0s;
    }

    video {
      transform: translate(-50%, -50%);
    }
  }
}

video,
img {
  max-width: 100%;
  height: auto;
}

.ui-nav {
  grid-row: 3 / 4;
  display: flex;
  flex-direction: column;
  align-self: end;

  > * {
    line-height: 1.5;
  }
}
!

JS 

              
                console.clear();

Array.from(
  document.querySelectorAll('.video-tile'),
  function(el){
    const video = el.querySelector('video');

    el.addEventListener('mouseenter', () => { 
      video.play()
    });

    el.addEventListener('mouseleave', () => {
      video.pause();
      setTimeout(() => { video.currentTime = 0 }, 400);
    });
  });

const elVideoToggle = document.querySelector('.video-toggle');

elVideoToggle.addEventListener('click', e => {
  elVideoToggle.classList.toggle('active');
  const condition = elVideoToggle.matches('.active');
  elVideoToggle.querySelector('video')[ condition ? 'play' : 'pause' ]();
});
!
999px

Console