UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36

Steps to reproduce the problem:
No steps, just a suggestion to add support for the safe-area-inset constant.

What is the expected behavior?
It would useful to have this with a set number of presets. So to start with the iPhone X.  Then as more devices or browsers use them more presets can be added. The ability to manually adjust these (only in DevTools) would also be handy.

What went wrong?
Nothing, just a suggestion.

Did this work before? N/A 

Chrome version: 60.0.3112.113  Channel: n/a
OS Version: OS X 10.12.6
Flash Version: 

How awesome would this be?!   ;-)