Yup. This is a nice (deliberate) side effect of using separate <aside> blocks instead of jamming them in the middle of the paragraph. The original location of the <aside> tag determines where the sidebar appears in the text flow if there's no room for it on the side. In some cases, it may be a few paragraphs distant from where it shows up on the side if that leads to better reading.
This is also the fallback for users who don't have JS enabled. (Well, all both of them, at this point.)
> I really love using the Mobile Design View in Firefox and Chrome for inspecting responsive behavior.
Oh, yeah. I basically lived in that for a few days when I was tweaking the CSS.
What help. /s
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.)
Searching "css sidenote edward morbius" on G+ might turn up something and basic CSS.
(Aside: I really love using the Mobile Design View in Firefox and Chrome for inspecting responsive behavior. You can drag the viewport size by hand, or switch it to presets that represent target devices easily. All while keeping the full width of the browser pane for developer/inspector UI. FF even has a nice toggle shortcut, ⌥⌘M on OS X.)