That's much better than the "superellipse" formula that was going around a few weeks ago. It was a good try, but it was pretty imprecise (based on an image that was too low-res to accurately compare).
Nice! One minor tweak that reduces redundancy in the CSS: You can set 'background-color: inherit' on the :before and :after pseudo-classes for the icon links and it works the same way without having to apply the background-color on the element and it's :before/:after.
As someone who is learning front-end, the way this tutorial presented is really valuable to me. Standard tutorials too often miss the 'whys' for your actions, and more. It's great to see the decision making and fine tuning done live.
Mouseover effects are neat for mouse users. I see on mobile the mouseover happens on touch/press. Assuming your icons are linking somewhere, is this useful for mobile or just for mouse folks?
I don't think I know anyone who wouldn't just call them rounded corners. Except maybe blogs trying to coin a new term to make them seem interesting again.
So in Chrome for Mac the icons glitch with the pinch effect but it works fine on Chrome for Windows. Why is the difference between the two Chromes not more acknowledged?
Edit: inserted bracketed qualifier after reading cheeaun's article.