<video id="video" width="320" height="240" controls>
  <source src="https://www.w3schools.com/howto/rain.mp4" type="video/mp4">
</video>
<a href="#" onclick="openFullscreen();">fullscreen</a>

var elem = document.getElementById("video"); 


function openFullscreen() {
  if (elem.requestFullscreen) {
    elem.requestFullscreen();
  } else if (elem.mozRequestFullScreen) { /* Firefox */
    elem.mozRequestFullScreen();
  } else if (elem.webkitRequestFullscreen) { /* Chrome, Safari and Opera */
    elem.webkitRequestFullscreen();
  } else if (elem.msRequestFullscreen) { /* IE/Edge */
    elem.msRequestFullscreen();
  }
}