HTML/CSS as in the example would indeed be usable, though a similar version of this (with inline labels and checkboxes used for "aside" notes) gets rather messy when used in a textual browser or converted into another format. JS-based expansion doesn't have to get unusable either (it can merely hide bits of a document after loading, with those present in HTML), but perhaps could become unusable more easily.

I'm not arguing for use of JS, by the way (not sure if it was clear), but rather for proper use of HTML.