Thanks for the links. The error does not seem to be of much concern to me. I checked the default Twenty Fourteen Theme and it has the same issue and hence I assume there are millions of websites out there with this warning.
However, if you need to get rid of it, you can make a small code change. Pls edit post-functions.php line number 42 from
$author = '' . __('Author: ', 'mo_theme') . '' . get_the_author_meta('display_name') . '';
to
$author = '' . __('Author: ', 'mo_theme') . '' . get_the_author_meta('display_name') . '';
Basically you need to remove the extra n from the class name. That should make it all “clean”.