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

The W3C HTML Validator checks that URLs provided in href attributes conform to the URL specification. Square brackets ([ and ]) are reserved characters with very specific, limited uses in URLs — they are only permitted in the host portion of a URL to denote IPv6 addresses (e.g., http://[::1]/). When they appear elsewhere, such as in the scheme data, path, or query string without being percent-encoded, the URL is considered malformed.

This commonly happens in a few scenarios:

• Mailto links where someone wraps an email address in brackets, like mailto:[user@example.com].
• Template variables that haven’t been processed, leaving literal bracket syntax (e.g., {{, [, ]) in the rendered HTML.
• Manually constructed URLs where brackets are mistakenly used as part of the path or query string instead of being percent-encoded as %5B and %5D.

Using invalid URLs can cause browsers to misinterpret the link destination, break navigation, or cause unexpected behavior. Assistive technologies such as screen readers also rely on well-formed URLs to correctly communicate link targets to users. Keeping your URLs standards-compliant ensures consistent, predictable behavior across all browsers and devices.

How to fix it

1. Remove unnecessary brackets. If the brackets are not part of the actual data (e.g., decorative brackets around an email address), simply delete them.
2. Percent-encode brackets when they are part of the data. If you genuinely need square brackets in a URL’s path or query string, encode [ as %5B and ] as %5D.
3. Check your templating engine output. If you use a templating system, inspect the final rendered HTML in a browser to make sure template syntax has been fully replaced with valid values.

Examples

Invalid: square brackets in a mailto URL

The brackets around the email address are not valid URL characters in this context.

<a href="mailto:[user@example.com]">Email Us</a>

Fixed: remove the brackets

<a href="mailto:user@example.com">Email Us</a>

Invalid: square brackets in a query string

<a href="https://example.com/search?filter[status]=active">Search</a>

Fixed: percent-encode the brackets

<a href="https://example.com/search?filter%5Bstatus%5D=active">Search</a>

Invalid: unprocessed template syntax in rendered HTML

If your templating engine fails to replace a variable, the final HTML may contain bracket characters:

<a href="mailto:[% user.email %]">Email Us</a>

Fixed: ensure the template renders a valid URL

Make sure the template variable resolves correctly. The rendered output should look like:

<a href="mailto:user@example.com">Email Us</a>

In your template source, this might be written as:

<a href="mailto:{{ user.email }}">Email Us</a>

The key is that the final HTML delivered to the browser must contain a valid, bracket-free URL (unless the brackets are properly percent-encoded). Always validate your rendered output, not just your template source, to catch issues like this.

Learn more:

MDN Reference

MDN - The Anchor element

→

MDN Reference

MDN - What is a URL?

→

WHATWG Spec

WHATWG HTML - Links

→