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

A [ in the fragment of a link's URL, the part after #, has to be percent-encoded as %5B.

The URL Standard only allows square brackets around an IPv6 address in the host, as in http://[::1]/. Anywhere else, including the fragment, a valid URL writes [ as %5B and ] as %5D. Browsers still follow a link with raw brackets, but the href value isn't a valid URL, and the validator reports the first [ it finds.

This often comes from long links copied out of a browser's address bar, such as map or search pages that keep coordinates or filters after the #. When the URL is that long (around 200 characters or more), the validator leaves it out of the message, which then reads Bad value for attribute "href" with no value at all. To find the link, look for an href with a [ after its #.

Encoding the brackets doesn't break links to elements on the same page. Browsers percent-decode the fragment when they look for a matching id, so href="#step%5B2%5D" still scrolls to id="step[2]". For a link to another site, open the encoded URL once to check that the target page still reads its fragment correctly.

Invalid example

<a href="https://example.com/map#view:[40.64,-8.63]">Open the map</a>

Valid example

<a href="https://example.com/map#view:%5B40.64,-8.63%5D">Open the map</a>

Learn more:

MDN Reference

MDN Web Docs: The a element

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MDN Reference

MDN Web Docs: URI fragment

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WHATWG Spec

URL Living Standard

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