I agree with Peter here.
Marginal notes, sidepar, changepar, etc should be treated analogously,
without undue emphasis on how a visual formatter displays them.
The fact that a footnote is printed at the bottom of a page, and a marginal
note printed on the margin is more a visual layout decision, rather than any
significant difference in the logical structure represented by these
constructs.
For instance, when AsTeR renders footnotes, I had a rendering rule that
resulted in the footnotes being spoken on the right speaker alone (AsTeR uses
stereo) at a slightly faster speech-rate. In a way, you could then say that
footnotes went into the margin.
The advantages of encoding constructs like sidepar ina layout independent way
are numerous; a browser can choose to render the object at a time most
convenient to the user.
For example, AsTeR provided a "float" rendering rule for footnotes, which
allowed footnotes to "float" to the end of the rendering of the enclosing
paragraph, section, or chapter, (with this choice entirely left to the
listener).
If a footnote or sidepar were to be too closely "bound" to the point of the
text where they occur, such flexible rendering would be impossible.