Interesting, the css fails to load for me.

It's the typical overly "rich" parallax-scrolling-images-behind-the-text stuff, though now some of them are videos, too, and sometimes they stay anchored for a while, and sometimes they play at the speed you are scrolling the text, for some reason.

I find it incredibly distracting, personally.

Agree, yes it might 'wow' you at first, but when you see this used more and more on other sites, the novelty is gone.

What does it really add, other than slowing down your scrolling (my laptop: i7, discrete gfx) and briefly distracting you while you focused on the text.

Yeah, same here. It was HTTPS everywhere's fault (or, rather, their fault for not supporting HTTPS correctly ;)