“&” did not start a character reference. (“&” probably should have been escaped as “&”.)

Unescaped & characters in HTML content or attribute values must be written as &.

In HTML, the & character signals the start of a character reference (like &, <, or ©). When the parser encounters a bare & that isn't followed by a valid reference, it produces a parse error. Browsers usually recover and display the & as intended, but the markup is technically invalid.

This most often appears in URLs with query string parameters (e.g., ?page=1&sort=asc) or in visible text like "Tom & Jerry". In both cases, every literal & must be escaped as &.

HTML examples

Invalid

<!-- Bare & in an href -->
<a href="search?category=books&sort=title&page=2">Results</a>

<!-- Bare & in visible text -->
<p>Fish & Chips</p>

Valid

<!-- Escaped & in an href -->
<a href="search?category=books&amp;sort=title&amp;page=2">Results</a>

<!-- Escaped & in visible text -->
<p>Fish &amp; Chips</p>

The rendered output is identical in both cases. Browsers display & as & to the user, and links work correctly with & in href values. The HTML parser converts it back to a literal & before making the HTTP request.

Learn more:

WHATWG Spec

Character references - HTML spec

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