I try to display a YouTube embed URL in a lightbox. This results in the following:

Refused to display 'https://www.youtube.com/' in a frame because it set 'X-Frame-Options' to 'sameorigin'.

What I find interesting is that data-preview-link="https://youtu.be/embed/U1t-LzSyH4Y" (youtu.be), while the console complains about https://www.youtube.com/ (www.youtube.com) refusing to connect:

When rendering a YouTube <iframe> myself, there doesn't seem to be a problem:

So it seems that the Reveal.js mechanism misses something necessary when creating the <iframe> by itself, so that YouTube is not satisfied with it and refuses any connection.