W3School TIY Editor

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

<h1>通过 JavaScript 实现全屏</h1>

<p>请单击按钮以全屏模式打开视频。</p>
<button onclick="openFullscreen();">以全屏模式打开视频</button>
<p><strong>提示：</strong>请按“Esc”键退出全屏。</p>

<video width="100%" controls id="myvideo">
  <source src="/i/video/shanghai.mp4" type="video/mp4">
  您的浏览器不支持 video 标签。
</video>

/* 打开全屏模式的功能 */
function openFullscreen() {
  /* 如果全屏模式可用，则全屏显示元素 */
  if (
    document.fullscreenEnabled || /* 标准语法 */
    document.webkitFullscreenEnabled || /* Safari */
    document.msFullscreenEnabled /* IE11 */
  ) {
   
    /* 全屏显示元素 */
    if (elem.requestFullscreen) {
      elem.requestFullscreen(); /* 标准语法 */
    } else if (elem.webkitRequestFullscreen) { /* Safari */
      elem.webkitRequestFullscreen();
    } else if (elem.msRequestFullscreen) { /* IE11 */
      elem.msRequestFullscreen();
    }
  }
}
</script>

<p>注释：Internet Explorer 10 及更早版本不支持全屏模式。</p>

</body>
</html>