UserAgent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36

Example URL:
any page with html5 video (such as:  https://www.quirksmode.org/html5/tests/video.html )

Steps to reproduce the problem:
1. find a html5 video (such as:  https://www.quirksmode.org/html5/tests/video.html ), enter fullscreen mode
2. use tab key navigation, the focus will go through the control bar and go to other elements in the page

What is the expected behavior?
the foucs should not go out of the fullscreen element

What went wrong?
Even there is a fullscreen element, I still can navigation out of it, and at the same time fullscreen mode dose not exit.

Does it occur on multiple sites: Yes

Is it a problem with a plugin? No 

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 63.0.3239.132  Channel: n/a
OS Version: 6.1 (Windows 7, Windows Server 2008 R2)
Flash Version: 

I have not found spec for this scenario, but seems a little troubling, right?
Is there any document for reference?
my chrome vesion is: 63.0.3239.132