UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.133 Safari/537.36

Steps to reproduce the problem:
CSS Values & Units level 4 defines the toggle() function. I am not sure how much is the priority on implementing it or even making it to Recommendation.

Creating this bug to track when it does get implemented, if ever.

What is the expected behavior?

What went wrong?
https://drafts.csswg.org/css-values-4/#funcdef-toggle

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 57.0.2987.133  Channel: n/a
OS Version: OS X 10.12.4
Flash Version: