Ah, more like a call-out box then. I was going to say earlier, but forgot to mention to, that if you really did mean a margin note you should try and work the CSS so that it is responsive and displays more like what your screenshot has on very narrow readers, like phones.
I also better understand what you mean about using a table—as I was trying to imagine juggling main body content and sidebar content in two cells and that did not sound fun at all. Even so, this type of tool is decidedly not what a table is meant to be used for, so it is better to stick with the solution above, just with slightly different CSS.
Shaded boxes like this are much easier to do, as floats can be a bit of “fun” to work with on ebooks (different platforms handling measurements slightly differently can make things like a consistent drop-cap, which would also use a float) tough to pull off well. But a shaded box is just a background-color, border and maybe some margin and padding to keep the text from bumping into it, both inside and out.
You could still use the aside element for that. There is nothing that states that has to be laid out a certain way, even if sidebar/margin-note presentation is most common. All it really means is something closer to the parenthetical, on a larger scale. That is what is meant, earlier, by calling it a more semantic use of HTML. We are saying what the text is, not brute forcing it with general-purpose layout constructs.
You can of course do all of that (and far more) with HTML, but I take your point. So given that, I would go with my advice to create a “Start Box” and “End Box” type style setup, around normally formatted text.
Here is an updated example of the first project. To see the new method in play (I left the old example in “Red Book”), flip to the first subdocument of “Black Book”. Note that whatever you type into the styled text itself is completely arbitrary because I’ve set this up to enclose whatever you type into an HTML comment. Just don’t type -->
or you’ll break the comment because that is how you end one!
note_example-mk2.zip (188.6 KB)
I also classed the two aside elements. The first now uses the ‘margin’ class and the new one uses ‘callout’.