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

A backslash (\) is not a valid character in a URL fragment and must be replaced with a forward slash (/) or percent-encoded.

The href attribute on an <a> element must contain a valid URL according to the URL Living Standard. The fragment portion of a URL — the part after the # symbol — follows the same rule: it can contain most characters, but the backslash (\) is explicitly forbidden as a bare character.

This commonly happens when copying file paths from Windows, which uses backslashes as directory separators, and pasting them into an href. Browsers may silently convert \ to /, but the markup is still invalid.

To fix this, replace every \ with / in the URL. If for some reason you actually need a literal backslash in the fragment, percent-encode it as %5C.

HTML Examples

❌ Invalid: backslash in the fragment

<a href="page.html#section\one">Link</a>

✅ Fixed: use a forward slash or percent-encoding

<!-- Option 1: Replace with forward slash -->
<a href="page.html#section/one">Link</a>

<!-- Option 2: Percent-encode the backslash -->
<a href="page.html#section%5Cone">Link</a>

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 - Links

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