I can guarantee it's not something that Sigil creates (and I'd be willing to bet a whole bunch of money that it doesn't come from calibre either). It may
allow deprecated attribute names to exist, but it certainly doesn't
create any.
Search the markup produced by Word2CleanHtml, and I'd be awfully surprised if it wasn't there already. If so, that implies that it either comes from Word or Wordstar--and since I've never encountered it (or heard of others doing so) in Word's Filtered HTML output ....