Bad value X for attribute “src” on element “iframe”: Illegal character in query. Space is not allowed.

A space character in the src attribute of an iframe is not valid in a URL. Spaces must be percent-encoded as %20 to produce a legal URL.

URLs follow the syntax rules defined in RFC 3986, which does not allow literal space characters anywhere in a URI, including the query string portion (the part after ?). When the HTML validator encounters a raw space in a URL attribute like src, it flags the value as malformed.

This commonly happens when query parameter values contain readable text, or when a URL is copied from a browser's address bar where the browser displays decoded spaces for readability. The fix is to replace each space with %20.

Some developers use + instead of %20 for spaces in query strings. The + convention originates from the application/x-www-form-urlencoded format used in HTML form submissions, and many servers accept it. However, %20 is the correct encoding according to RFC 3986 and will pass W3C validation without issues.

HTML examples

Invalid: space in the query string

<iframe
  src="https://example.com/embed?title=my page&lang=en"
  width="600"
  height="400">
</iframe>

Valid: space encoded as %20

<iframe
  src="https://example.com/embed?title=my%20page&lang=en"
  width="600"
  height="400">
</iframe>

Learn more:

MDN Reference

MDN: iframe element

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MDN Reference

MDN: What is a URL?

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WHATWG Spec

WHATWG: The iframe element

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