W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>

<h2>Fullscreen with JavaScript</h2>
<p>Click on the button to open the video in fullscreen mode.</p>
<button onclick="openFullscreen();">Open Video in Fullscreen Mode</button>
<p><strong>Tip:</strong> Press the "Esc" key to exit full screen.</p>

<video width="100%" controls id="myvideo">
  <source src="rain.mp4" type="video/mp4">
  <source src="rain.ogg" type="video/ogg">
  Your browser does not support the video tag.
</video>

/* Function to open fullscreen mode */
function openFullscreen() {
  /* If fullscreen mode is available, show the element in fullscreen */
  if (
    document.fullscreenEnabled || /* Standard syntax */
    document.webkitFullscreenEnabled || /* Safari */
    document.msFullscreenEnabled /* IE11 */
  ) {
   
    /* Show the element in fullscreen */
    if (elem.requestFullscreen) {
      elem.requestFullscreen(); /* Standard syntax */
    } else if (elem.webkitRequestFullscreen) { /* Safari */
      elem.webkitRequestFullscreen();
    } else if (elem.msRequestFullscreen) { /* IE11 */
      elem.msRequestFullscreen();
    }
  }
}
</script>

</body>
</html>

​x
 
<!DOCTYPE html><html><head><meta name="viewport" content="width=device-width, initial-scale=1"></head><body>​<h2>Fullscreen with JavaScript</h2><p>Click on the button to open the video in fullscreen mode.</p><button onclick="openFullscreen();">Open Video in Fullscreen Mode</button><p><strong>Tip:</strong> Press the "Esc" key to exit full screen.</p>​<video width="100%" controls id="myvideo">  <source src="rain.mp4" type="video/mp4">  <source src="rain.ogg" type="video/ogg">  Your browser does not support the video tag.</video>​<script>/* Get the element you want displayed in fullscreen */ var elem = document.getElementById("myvideo");​/* Function to open fullscreen mode */function openFullscreen() {  /* If fullscreen mode is available, show the element in fullscreen */  if (    document.fullscreenEnabled || /* Standard syntax */    document.webkitFullscreenEnabled || /* Safari */    document.msFullscreenEnabled /* IE11 */  ) {       /* Show the element in fullscreen */    if (elem.requestFullscreen) {      elem.requestFullscreen(); /* Standard syntax */    } else if (elem.webkitRequestFullscreen) { /* Safari */      elem.webkitRequestFullscreen();    } else if (elem.msRequestFullscreen) { /* IE11 */      elem.msRequestFullscreen();    }  }}</script>​</body></html>​