Unmatched or missing quotes on a class name cause various problems on W10 with Sigil 2.3.1 and on MacBook with 2.3.0
<p class=Textbody"> is picked up by the QtWebengine although as noted above in this thread its line/column location is a bit doubtful.
But it is not picked up by Sigil's 'Well-Formed Check EPUB' tool although <p class="Textbody> is caught by Well-Formed Check
Missing quotes <p class=Textbody> is not picked up by Well-Formed Check
<p class='Textbody"> These unmatched quotes are picked up by Well-Formed Check and QtWebengine.
And most curious of all for me is that a single occurence of one of these "quotes" examples produces a whole range of "Fragment identifier not defined" errors by the EpubCheck plugin with references to various pages, lines, and columns. These fragments are nowhere in the vicinity of the missing or unmatched quotes which the plugin does correctly locate.
As soon as the "quotes" error is fixed, a rerun of the EpubCheck plugin finds no faults at all.
So fixing a missing quote cures undefined fragments - hard to believe.