@Override
    public void checkValid(CharSequence literal) throws DatatypeException {
        for (int i = 0; i < literal.length(); i++) {
            char c = literal.charAt(i);
            if (!isWhitespace(c)) {
                return;
            }
        }
        throw newDatatypeException("An IDREFS value must contain at least one non-whitespace character.");
    }
Checks that the value is a proper list of HTML5 ids.
@param literal the value
@throws DatatypeException if the value isn't valid
@see org.relaxng.datatype.Datatype#checkValid(java.lang.String, org.relaxng.datatype.ValidationContext)
src/nu/validator/datatype/Idrefs.java