New epub file. I enter this code. I save the file, refusing the offer to fix it. Preview catches the error. FlightCrew catches the error. EpubCheck returns 'No problem found'.
Windows 10. Sigil 0.9.18. EpubCheck 4.22.
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
</head>
<body>
<p>Test</p>
<p>Test</div>
</body>
</html>