Bad value X for attribute “href” on element “a”: Illegal character in query. Y is not allowed.

A URL in an href attribute contains a character that must be percent-encoded before it can appear in a query string.

URLs follow the syntax rules defined in RFC 3986. The query component of a URL (everything after the ?) allows a specific set of characters: unreserved characters (A-Z, a-z, 0-9, -, ., _, ~), sub-delimiters (!, $, &, ', (, ), *, +, ,, ;, =), plus :, @, /, and ?. Characters outside this set, such as {, }, |, ^, [, ], spaces, and non-ASCII characters like é or ñ, are illegal in their raw form and must be percent-encoded.

Percent-encoding replaces each disallowed byte with a % followed by its two-digit hexadecimal value. For example, a space becomes %20, a pipe | becomes %7C, and { becomes %7B.

Common causes of this error include:

• Pasting a URL directly from a browser address bar or CMS that displays decoded characters.
• Using curly braces for template placeholders in URLs without encoding them.
• Including spaces or non-ASCII characters in query parameter values.

HTML examples

Invalid: raw illegal characters in the query string

<a href="https://example.com/search?q=hello world&cat=sci|tech">Search</a>

The space between hello and world and the pipe | between sci and tech are not allowed in a URL query.

Valid: percent-encoded characters

<a href="https://example.com/search?q=hello%20world&cat=sci%7Ctech">Search</a>

Each illegal character is replaced with its percent-encoded equivalent: space is %20 and | is %7C.

Learn more:

MDN Reference

MDN: The Anchor element

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MDN Reference

MDN: What is a URL?

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WHATWG Spec

WHATWG HTML Standard: Links

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