On Mac OS X 10.11.2, Chrome Version 47.0.2526.111 (64-bit)

When trying to watch the YouTube video embedded on this page https://www.ampproject.org/docs/get_started/about-amp.html the video doesn't actually go full screen.

Here's a screenshot of what the video looks like in full screen.

Full screen YouTube works fine on the main site and other embeds on third party sites pointing toward an issue with the amp-youtube element.