Hallo Matthias Apsel,

Die Spezifikation (der WHATWG) ist sogar noch ein wenig strenger:

An ambiguous ampersand is a U+0026 AMPERSAND character (&) that is followed by one or more alphanumeric ASCII characters, followed by a U+003B SEMICOLON character (;), where these characters do not match any of the names given in the named character references section.

Bis demnächst
Matthias