"Display videos from YouTube" instructions don't work?

I added the (exact) code from Display videos from YouTube into my README.md file, being careful to leave a blank line above and below as described, and even used the same YouTube URL as in the docs…

…and nothing appears. There is no video embedded. What’s wrong?
Thanks.

(I came to GitLab because GitHub was not rendering videos, and saw the aforementioned docs, so was encouraged about being able to do this…)