The above html will show the video in a message, of course, but you can also select 'Copy video url' from the same menu (or just grab it from the browser window if you're on Youtube), and if you dump it into the message like so:
it'll render as the video. If ever in doubt about whether or not it'll show up, just hit 'Preview' to check.
The above example is for a Jane's Addiction performance on Jools Holland's show a few years back that I've really been digging lately, so to share it with you all, I just cut and paste the url into the msg and voila:
Whether it's the drop D groove to the little Lydian-flavored bridge, I hope you enjoy!