All solutions found using Google don't work for me. Can someone show me a simplest possible .Rmd
file that embeds a YouTube video?
According to R Markdown: The Definitive Guide something as simple as 
should work. This post on SO tells me to use:
<iframe width="560" height="315" src="https://youtu.be/zNzZ1PfUDNk" frameborder="0" allowfullscreen></iframe>
My simple .Rmd
file with only these two snippets looks like this:

<iframe width="560" height="315" src="https://youtu.be/zNzZ1PfUDNk" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
But knitted to an HTML I only get this:
What am I missing here?