[html5] Validator.nu’s (X)HTML5 Validator does not validate XHTML5

Mon Aug 28 03:04:43 PDT 2017

Validator.nu (X)HTML5 Validator at https://html5.validator.nu
validates only HTML5, not XHTML5, despite its name, when attempting to
do so via direct input (Text Field).

Tested with this markup:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
   <head>
      <meta charset="UTF-8"/>
      <title>My title</title>
   </head>
   <body>
   </body>
</html>

The submission form does not have an option to forcibly request XHTML5
validation instead of HTML5. Even when the markup is preceded by the
XML declaration (which nevertheless is purportedly not necessary if
contents are UTF-8), the validator reports the very declaration as an
error:

> Error: Saw <?. Probable cause: Attempt to use an XML processing instruction in HTML. (XML processing instructions are not supported in HTML.)

revealing that even with such markup it is validating HTML5 instead of XHTML5.

The generic validator at https://validator.nu used to have a preset or
parser (cannot remember) specifically for XHTML5, but not anymore
either it seems.