此代码不依赖CSS文件：

<div style="position:relative;padding-top:56.25%;">
   <iframe src="https://www.youtube.com/embed/Nf6MGedTo2c" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe>
</div>

参考文章：
How do I make a full-width iframe with fixed aspect ratio?