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).