Related, I just found out that Darcy Clarke put this list together of good front-end interview questions. I would have included it in the article if I'd known about it ahead of time.
The ability to emulate different versions of IE. Of course, the only reason this is necessary is because of the many breaking changes that the different versions created.
I remember coming across this other "Learn Front-end" list: https://github.com/dypsilon/frontend-dev-bookmarks on HN a little while ago and thought it was a great example of just how scattered Web-FE education still is. Everyone has their own favorite "book" and there's no K&R that you just pick up and read.
I think I see what she did there..