[vsbuffalo]

Well done. Though shouldn't the sidenotes use HTML5's <aside></aside>? How does the vertical alignment of sidenote callout and sidenote work?

[exupero]

I agree.

It looks like sidenotes are floated right and given negative right margin. (https://github.com/daveliepmann/tufte-css/blob/master/tufte....)

[dredmorbius]

aside doesn't position relative to text.

There really ought to be a native HTML reference / footnote / sidenote entity with appropriate default styling.

My take, responsive:

http://imgur.com/a/TXpis

(Using floats and negative margins.)