<com:TForm ID="form1"> <h1>Callback Enabled Calculator</h1> <com:TActiveTextBox id="a" /> + <com:TActiveTextBox id="b" /> = <com:TActiveTextBox id="c" /> <com:TActiveButton id="sum" onclick="do_sum" text="Calculate!" /> <com:TRequiredFieldValidator ControlToValidate="a" ErrorMessage="left summand is required." ControlCssClass="required" Display="None" /> <com:TRequiredFieldValidator ControlToValidate="b" ErrorMessage="right summand is requied." ControlCssClass="required" Display="None" /> <div class="summarybox"> <com:TValidationSummary ID="summary" HeaderText="Unable to calculate because" /> </div> </com:TForm>