Bad value “mailto: X” for attribute “href” on element “a”: Illegal character after “ilto:”. Space is not allowed.

A mailto: link must not contain spaces between the colon and the email address.

The href attribute on an <a> element accepts mailto: URLs as defined in RFC 6068. The email address must immediately follow the colon with no whitespace. Browsers may still attempt to open the mail client, but the space makes the URL technically invalid and triggers a W3C validation error.

This applies to all parts of the mailto: URL. Spaces before the address, between multiple addresses, or before query parameters like ?subject= are all invalid. If you need to include a space in a subject or body parameter, encode it as %20.

HTML examples

Invalid

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

Valid

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

Learn more:

MDN Reference

HTML anchor element: href attribute

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WHATWG Spec

HTML links and forms

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MDN Reference

mailto: URL scheme

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