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

An unescaped & in an attribute value is followed by the name of a character reference, so the parser turned that text into a different character.

HTML treats & as the start of a character reference such as &, © or ®. A few older names, including copy, reg, para and not, still work without the trailing semicolon. Inside an attribute value, the parser converts one of them when the next character isn't =, a letter or a digit, or when the value ends right after the name. So title="&copy 2025 Example News" becomes "© 2025 Example News", and href="/news?id=10&para" ends in ¶ instead of &para, which sends the link somewhere else.

When the name is followed by =, a letter or a digit, as in ?id=10&reg=europe, the parser leaves the text alone so that old URLs keep working, and the validator doesn't report it. In regular text outside attributes, a reference without its semicolon gets a different message: Named character reference was not terminated by a semicolon.

Escape every literal & as &. If you meant the symbol, end the reference with a semicolon.

Invalid example

<!-- &para becomes ¶ and &copy becomes © -->
<a href="/news?id=10&para" title="&copy 2025 Example News">Read the report</a>

Valid example

<a href="/news?id=10&amp;para" title="&copy; 2025 Example News">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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