[KevinH]

I took a look at the Sigil 0.8 branch which was before any epub3 support was added to Sigil at all, and the code back at the height of epub2 used the u tag and epubcheck at the time and our own syntax checker Flightcrew (now retired) had no problems with it at all.

My guess is this is an epubcheck bug being overly literal with changes from Xhtml 1.0 to XHtml 1.1 when the u tag was deprecated but never removed.

Also not sure how maintained the epub2 specific code in epubcheck really is anymore? My guess is no one there really cares about epub2 anymore when they are resource constrained just trying to support epub3.

So maybe just bitrot?

So if newer epubcheck now objects to the u tag under epub2, and you are constantly editing epub2s, I would just use my own clip.