It's a visual thing. Frontend isn't very similar to programming for the backend (except for high quality javascript). Most people who can do wonders for frontend tend to be more like designers.
With all the CSS transforms, animations, canvas, 3d libraries, etc., I'm now utterly lost. I've gone from being an intermediate frontend guy to a 6/10 on the core necessities (all that earlier knowledge is still the foundation), but like a 1/10 or 2/10 with all the new stuff. I could probably get to 4/10 if I put in the effort, but I have a very strong gut feeling that I simply do not have the... faculties... required to be an intermediate frontend guy in 2015.
All that to say... codepens like these I've seen blow me away. I don't even bother trying to dig into the CSS. It's just beyond me. ;)