The string following “&” was interpreted as a character reference. (“&” probably should have been escaped as “&”.)

When the characters right after an unescaped & spell a known entity name, the parser reads them as a character reference instead of literal text.

HTML treats & as the start of a character reference such as &, ©, or ®. Many of these references are also recognised without their trailing semicolon, so a bare & followed by a name like reg, copy, or para gets converted to ®, ©, or ¶. The validator reports this because the result is almost never what the author intended.

This usually bites in URL query strings, where parameter names happen to match entity names. In ?id=1&reg=eu, the &reg turns into ®, so the link points somewhere different from what the source shows. Escape every literal & as & to keep the text intact.

HTML examples

Invalid

<!-- &reg becomes ® and &para becomes ¶ -->
<a href="/news?id=10&reg=europe&para=3">Read the report</a>

Valid

<!-- The & characters are kept literal -->
<a href="/news?id=10&amp;reg=europe&amp;para=3">Read the report</a>

Browsers convert & back to a single & before requesting the URL, so the link works exactly as written while the markup stays valid.

Learn more:

WHATWG Spec

Character references - HTML spec

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