I had this problem.

I wanted to embed a YouTube video in a page, but since you need to use an iframe I couldn’t figure out how to properly set the height and width for it, in a way that would work on a fluid layout.

After a while I found the solution

Tailwind code with React:

<iframe className="aspect-video w-full"
  src={"Youtube embed URL"}>
</iframe>

Tailwind code without React:

<iframe class="aspect-video w-full"
		    src="Youtube embed URL">
</iframe>

Plain HTML and CSS

<iframe style="aspect-ratio: 16 / 9; width: 100%"
  src="YouTube embed URL"></iframe>

The YouTube embed URL is something like

https://www.youtube.com/embed/VIDEO_ID

So if you have the video URL you must change that, for example with

videourl.replace('https://www.youtube.com/watch?v=', 
  'https://www.youtube.com/embed/')

Some old tutorials still list the absolute/relative trick, like this:

<style>
.videocontainer {
	position:relative; 
	padding-bottom:56.25%;
}
.videocontainer iframe {
	width:100%;
	height:100%;
	position:absolute;
}
</style>

<div class="videocontainer">
  <iframe src="YouTube embed URL"></iframe>
</div>

I prefer the simpler aspect-ratio property.