[KevinH]

Yes, Xhtml spec requires all attributes to be quoted - either with single or double quotes.

If they had run Sigil's well formed check, it should detect things like unopened or unclosed attributes and warned them but sometimes if a suitable quote is found "nearby", even that gets confused.

The missing end quote means the attribute value actually encompassed the closing > and converted it to & lt ; to prevent its loss since > are not allowed inside attribute quoted strings without them being xml encoded. It repeated that and converted bare text to attributes and ended that attribute later.

The missing quotes around an attribute are really hard to detect at times and this was one of them. Luckily, all text has been either encoded, made into empty attributes or saved in some way. So you can fix it by hand.

That said, gumbo follows the same whatwg spec for xhtml so Preview was seeing that code and internally doing pretty much the same thing as gumbo showed.

Please try Sigil's built in well formed check or epubcheck on the initial broken attribute code to see if either would better warn you of the breakage properly.