You may try
TidyGUI for Windows users to get rid of those '?'. Open your Source file, click the 'Tidy!' button, then click the 'Show Output' button and 'Save as...'.
You can change the TidyGUI
defaults by clicking the 'Configuration' button and then under the 'Encoding' tab choose the options that suit your needs.
Also, you may reference the
Special Character Set at the eBook Technologies, Inc. website for which special characters are available to use in all eBook Editions.
For an example how these special characters look on the EBW 1150, see the second attachment,
REBTestDocument_x-small.imp and go to page 17 to 25. The columns represent two entries per line of the "
Char" "
HTML code" and "
Numeric Code" for the special character.