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The main problems with that (aside from epub content is (a subset of) XHTML1.1, not HTML5 -- but span is in former, so it makes no difference): it doesn't get displayed at all, and it can't contain markup if it did. The same place also suggests using links with square brackets around the numbers: that sucks, too (presumably it's non-normative). The use of a well-defined "rel" attribute allows you to put the square brackets in, if that's what you want, in the style sheet -- while also allowing it to be formatted in other ways (e.g., as a superscript) if you prefer, without changing the source document. Putting square brackets or other markup in the source makes that impossible.
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