Bad value for attribute “href” on element “a”: Illegal character in fragment: “|” is not allowed.

A | (pipe) character in the fragment portion of a URL must be percent-encoded as %7C to be valid.

The href attribute on an <a> element must contain a valid URL. According to the URL specification, certain characters — including the pipe | — are not permitted as literal characters in a URL fragment (the part after #). The W3C validator flags this because the browser may handle it inconsistently, and it violates the URL standard.

To fix it, replace every | in the URL with its percent-encoded equivalent: %7C.

HTML Examples

❌ Invalid: literal pipe in fragment

<a href="https://example.com/page#section|one">Link</a>

✅ Valid: percent-encoded pipe in fragment

<a href="https://example.com/page#section%7Cone">Link</a>

If you control the target page, consider redesigning the fragment identifiers to avoid special characters altogether — for example, using hyphens or underscores like #section-one instead.

Learn more:

MDN Reference

HTML a element - MDN

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MDN Reference

What is a URL? - MDN

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WHATWG Spec

URL Living Standard

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