Safari display:contents bug

I just came across an interesting bug. In Safari 11.0.2 on desktop and on the phone, display:contents falls back to display:inline instead of being ignored.

I created a Pen to demonstrate:

See the Pen display contents by Jonathan Snook (@snookca) on CodePen.

I also was able to test on iOS 10.3.3 and it also seemed to exhibit this behaviour. iOS 9 didn't.