First, the HTML 5 specification is constantly changing so validators and the validity of this answer can be expected to break.
That being said, I repeat the quote that defines an “ambiguous ampersand”:
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.
In other words it's something that looks like a named character reference but is unknown to the specification. Now that the specification defined the term it defines when such ambiguous ampersands must not occur:
textarea
, title
: Escapable raw text elements can have text and character references, but the text must not contain an ambiguous ampersand.
- MathML, SVG elements: … but the text must not contain the character U+003C LESS-THAN SIGN (<) or an ambiguous ampersand.
- Ordinary non-empty HTML elements: … but the text must not contain the character U+003C LESS-THAN SIGN (<) or an ambiguous ampersand.
- Attribute values: … with the additional restriction that the text cannot contain an ambiguous ampersand.
The bullet points have been quoted from the specification, too. Please search the specification for “ambiguous ampersand” for the full sentences that have been omitted here.
The HTML 5 specification does allow ambiguous ampersand in raw text elements (script
and style
tags), though. Just because HTML 5 it gives a definition for “ambiguous ampersands” and browsers are able to cope with “ambiguous ampersands” most times it does not mean they are valid for general use.
So do escape “ambiguous ampersands” to make them unambiguous except in script
and style
tags.
Let's come back to your case. You do not have an “ambiguous ampersand” because your ampersand is not followed by alphanumerics and a semicolon. As it is not followed by that sequence one should assume your ampersand is to be taken literally and retain it as is. Therefore your ampersand should be considered valid according to the HTML 5 specification.
Remark: I'd suggest to escape your ampersand nevertheless as your are relying on a detail of an unstable specification. Additionally I would not expect every software to follow the specification that close and instead go with the simpler rule to escape ampersands whenever they occur as I cannot see this calls for trouble.