Thanks, Chad, for your reply.
Yes, I did have both client-side & server-side validations ticked. As you've suggested, I've unticked the server-side option. However, I've looked over every single option & setting in the entire "Manage Dynamic Questions" edit interface, several times, but I just could not find the hide the validation summary option. Where exactly it is in Version 2.3 ?
As for the styles for the error messages, I've looked at the source code, and I've found that there is no style classes assigned to ValidationSummary--the style specifications are inline items, like this . . .