<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Full Screen</title>
  <style>
    #player:fullscreen #media {
      width: 100%;
      height: 100%;
    }
  </style>
  <script>
    let video, player;
    function initiate() {
      video = document.getElementById("media");
      player = document.getElementById("player");
      player.addEventListener("click", gofullscreen);
    }
    async function gofullscreen() {
      try {
        if (!document.fullscreenElement) {
          await player.requestFullscreen();
          video.play();
        } else {
          await document.exitFullscreen();
          video.pause();
        }
      } catch (error) {
        console.log("Error: " + error.name);
      }
    }
    window.addEventListener("DOMContentLoaded", initiate);
  </script>
</head>
<body>
  <section id="player">
    <video id="media" width="720" height="400" poster="poster.jpg">
      <source src="trailer.mp4" type="video/mp4">
      <source src="trailer.webm" type="video/webm">
    </video>
  </section>
</body>
</html>