I just ran into that same code on a non-wordpress site, (drupal), and it looks to me like it was via a highlighter plugin in Firefox. It was on a co-workers computer, and when she edited a page in a gui editor, it added that extra code, and underlined a couple of words with a yellow highlight. When I disabled the FF plugin, the code was on the same page, but it didn’t have the hover elements it had previously had.
Edit: https://addons.mozilla.org/en-US/firefox/addon/11808/ this is the addon that I had to remove.
