About This HTML Issue
A mailto: link in an href attribute contains a [ character that is not valid in a URI according to RFC 3986.
Square brackets ([ and ]) are reserved characters in URIs and must be percent-encoded when used outside their specific purpose (IPv6 address literals). If the email address or other part of the mailto: URI contains a [, it must be encoded as %5B. Similarly, ] must be encoded as %5D.
This error typically appears when a mailto: link includes square brackets in the subject, body, or display-related parts of the URL. Query parameters in mailto: links — such as ?subject= or ?body= — must have their values properly percent-encoded.
Invalid example
<a href="mailto:info@example.com?subject=[Action Required] Review">
Email us
</a>
Valid example
Percent-encode the square brackets in the query string:
<a href="mailto:info@example.com?subject=%5BAction%20Required%5D%20Review">
Email us
</a>
The [ becomes %5B, the ] becomes %5D, and spaces become %20. Most server side languages and JavaScript provide functions to handle this encoding automatically, such as encodeURIComponent() in JavaScript.
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