HTML Guide
This is a warning that a special character in the Unicode Private Use Area is being used at the document, which might cause it to not work the way you might expect in different browsers/environments.
If you’ve checked the document in different browsers and it’s working fine, you can safely ignore this warning.
What are private-use characters in Unicode?
Private-use characters are code points whose interpretation is not specified by a character encoding standard and whose use and interpretation may be determined by private agreement among cooperating users. Private-use characters are sometimes also referred to as user-defined characters (UDC) or vendor-defined characters (VDC).
Learn more:
Related W3C validator issues
Ensure you’re not using character references that expand to control characters, like , which are not permissible in HTML documents.
In HTML, a character reference allows you to use a specific ASCII or Unicode character in your document. Character references are written using the syntax &#code; where code is either the decimal or hexadecimal code point of the character. Control characters, like U+0002, are non-printable and are not allowed within HTML because they do not represent meaningful text content.
Character references should only be used for printable characters and standard entities. For example, common entities like & and < should be used for special characters like & and <.
Example of Incorrect Usage
The following example shows an HTML snippet where a control character is incorrectly referenced:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Example Page</title>
</head>
<body>
<p>Control character reference: </p>
</body>
</html>