unescaped & which should be written as &

Cause:

The ampersand ("&") is a special character in HTML. It marks the beginning of a entity, like "&nbsp" for a non-breaking space.

In XHTML, a entity must also end with a semicolon (";"). For example " "

Because this is so, any time a literal ampersand appears in a document, it needs to be written as a character entity, "&". Ampersands commonly appear in the query string of a URL, and need to be expressed as an entity there.

Example - Normal Ampersand:

GoodSmith & son
GoodSmith & son

Example - Ampersand in a Query String:

Good<a href="http://www.domain.com/cgi?x=1&y=2">query string</a>
Good<a href="http://www.domain.com/cgi?x=1&amp;y=2">query string</a>

Solution:

Please replace "&" with "&amp;".

References: