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.

Lessons in this unit:

0:	Introduction
1:	How to center an element with CSS
2:	CSS Border inside the element
3:	What are good CSS Breakpoint values for Responsive Design?
4:	How to debug CSS by bisecting
5:	How to disable text selection using CSS
6:	How to put an item at the bottom of its container using CSS
7:	CSS, how to select elements that do NOT have a class
8:	How to stick an element on the bottom of the page with flexbox
9:	How to apply padding to multiple lines in CSS
10:	Making a table responsive using CSS
11:	CSS url()
12:	How to make an element smaller or bigger with CSS
13:	Customizing visited links
14:	Fix extra space after inline element
15:	How to create a sidebar that’s sticky but also scrolls
16:	▶︎ How to embed YouTube videos using the correct aspect ratio
17:	Responsive pre tags in CSS
18:	Responsive YouTube Video Embeds
19:	How to remove all CSS from a page at once
20:	How I added Dark Mode to my website
21:	How to add a simple dark mode