It's not ready for prime time. I thought I'd posted some sample code. I'll dig for it. That was while I was doing anonymous codepens, so they're not on my profile there.

Searching "css sidenote edward morbius" on G+ might turn up something and basic CSS.

Basic sidenotes http://codepen.io/dredmorbius/pen/OVmKZX?editors=110

Not responsive, but @media queries on width and re-scaling width, margin, background colour, and border colour is what gets you that.

Motherfucking Website. No sidenotes. but other bones of my basic design. http://codepen.io/dredmorbius/pen/KpMqqB

Daniel Bos's sidenote implementation: http://loadingdata.nl/waves/

(Degrades nicely for non-CSS viewers, not responsive.)