SharePoint InputFormTextBox Validation

RequiredFieldValidator can not work with SharePoint InputFormTextBox. You have to client side JavaScript validation instead. There's an out-of-box function RTE_GetRichEditTextOnly come handy to retrieve the value from InputFormTextBox when doing JavaScript validation:

function ValidateInputFormTextbox() {
    var inputText = RTE_GetRichEditTextOnly("<%= inputControl.ClientID %>");
    if (inputText)
        return true;
    else
        return false;
}