About This HTML Issue
A space character in the href attribute of an <a> element is not valid in a URL. Spaces must be encoded as %20 in the query string.
URLs follow the syntax defined in RFC 3986, which does not allow literal space characters anywhere in the URL. When the W3C validator encounters a space in a query string, it flags the href value as malformed. Browsers often handle this gracefully by encoding the space automatically, but the HTML itself is still invalid.
The query component of a URL (everything after the ?) follows the same rule. If a value in the query string contains a space, replace each space with %20. An alternative encoding, +, is specific to the application/x-www-form-urlencoded format used by HTML form submissions, and is also accepted in practice by most servers. However, %20 is the universally correct percent-encoding for a space in any part of a URL.
Invalid example
<a href="https://example.com/search?query=hello world&lang=en">Search</a>
The space between hello and world causes the validation error.
Valid example
<a href="https://example.com/search?query=hello%20world&lang=en">Search</a>
Every space in the URL is replaced with %20, and the markup passes validation.
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