Well done. Though shouldn't the sidenotes use HTML5's <aside></aside>? How does the vertical alignment of sidenote callout and sidenote work?
It looks like sidenotes are floated right and given negative right margin. (https://github.com/daveliepmann/tufte-css/blob/master/tufte....)
There really ought to be a native HTML reference / footnote / sidenote entity with appropriate default styling.
My take, responsive:
(Using floats and negative margins.)