<div class="container">
  <p><em>Click the button below to enter the element into fullscreen mode. </em></p>
  <div class="element" id="element">
    <p>I'm in fullscreen mode!</p>
  </div>
  <br>
  <button onclick="var el = document.getElementById('element'); el.webkitRequestFullscreen();">Go Full Screen!</button>
</div>
body {
  color: #555;
  font-size: 1.1em;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
}

em {
  font-size: 0.9em;
}

.container {
  margin: 40px auto;
  max-width: 700px;
}

.element {
  padding: 20px;
  height: 300px;
  width: 100%;
  background-color: skyblue;
}

.element p {
  visibility: hidden;
  text-align: center;
  color: white;
  font-size: 3em;
}

.element:-webkit-full-screen {
  background-color: #e4708a;
  width: 100vw;
  height: 100vh;
}

.element:-webkit-full-screen p {
  visibility: visible;
}

.element:-moz-full-screen {
  background-color: #e4708a;
  width: 100vw;
  height: 100vh;
}

.element:-moz-full-screen p {
  visibility: visible;
}

.element:-ms-fullscreen {
  background-color: #e4708a;
  width: 100vw;
  height: 100vh;
}

.element:-ms-fullscreen p {
  visibility: visible;
}

.element:fullscreen {
  background-color: #e4708a;
  width: 100vw;
  height: 100vh;
}

.element:fullscreen p {
  visibility: visible;
}