自适应屏幕大小，代码如下

<div style="position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden;">
  <iframe width="100%" height="100%" src="https://www.youtube.com/embed/视频ID" frameborder="0" allowfullscreen style="position: absolute; top: 0; left: 0; width: 100%; height: 100%;"></iframe>
</div>

效果图：