The only built-in entities in XML are &, <, >, " and ' XHTML added the others via a DTD that is not a part of HTML5. As such, validators will report them as errors.
Safe replacements are the decimal notation:   or the character itself U+00A0;
Quite a few other common symbols are not available without similar changes.
<=<>=>&=&'='"=" – ©=©®=®™=™
REFERENCES: