如果播放器被包含在 iframe 里，尝试在 iframe 上添加 allowfullscreen 属性。
为了完善的浏览器兼容性，它应该是这样：
<iframe src="example.com" allowfullscreen="allowfullscreen" mozallowfullscreen="mozallowfullscreen" msallowfullscreen="msallowfullscreen" oallowfullscreen="oallowfullscreen" webkitallowfullscreen="webkitallowfullscreen"></iframe>