HTMNL Editor does not offer the HTML 5 option.
you will need to edit the code after it is generated and chacnge:
$CKEditor_config["docType"] = "<" ."!" ."DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">";
to:
$CKEditor_config["docType"] = "<" ."!" ."DOCTYPE html>";
NOTE, the doctype is only applied if you set the option for full page editing.