Client function doesn't works if you set the ASP.NET server control AutoPostBack to 'True'.

Acutually, the Client validate function also works as loog as the OnTextChanged event was fired. Only the Client validate works before the Post back caused by OnTextChanged.

In fact, when you drag the ASP.NET validator control to the page, when you run the page, you will find that the form has be added a property(Onsubmit) and assign 'javascript:return WebForm_OnSubmit();' to this property. Such as follows:

< form  name ="form1"  method ="post"  action ="Test6.aspx"  onsubmit ="javascript:return WebForm_OnSubmit();"  id ="form1" >   

Meanwhile, please go ahead and tip the related JavaScript function.

< script type = " text/javascript " >   
// <![CDATA[  
function  WebForm_OnSubmit() {  
if  ( typeof (ValidatorOnSubmit)  ==   " function "   &&  ValidatorOnSubmit()  ==   false )  return   false ;  
return   true ;  
}  
// ]]>  
< / script>  

And also please notice the __doPostBack function code segment which be generated due to the AutoPostBack property of TextBox has been set to 'True'.

function  __doPostBack(eventTarget, eventArgument) {   
     if  ( ! theForm.onsubmit  ||  (theForm.onsubmit()  !=   false )) {   
        theForm.__EVENTTARGET.value  =  eventTarget;   
        theForm.__EVENTARGUMENT.value  =  eventArgument;   
        theForm.submit();   
    }   
}  

By now, you will found that the __doPostBack function will be fired before the WebForm_OnSubmit() funtion. Hence, the situation has happend like you describled.
So, please refer to the following code to solve your problem.

< html  xmlns ="http://www.w3.org/1999/xhtml" >   
< head  runat ="server" >   
     < title ></ title >   
  
</ head >   
< body  >   
     < form  id ="form1"  runat ="server" >   
     < script  type ="text/javascript" >   
         function  __doPostBack(eventTarget, eventArgument) {   
             if  ( typeof  (Page_ClientValidate)  ==   ' function ' ) {   
                Page_ClientValidate();   
                 if  (Page_IsValid)   
                     if  ( ! theForm.onsubmit  ||  (theForm.onsubmit()  !=   false )) {   
                    theForm.__EVENTTARGET.value  =  eventTarget;   
                    theForm.__EVENTARGUMENT.value  =  eventArgument;   
                    theForm.submit();   
                }   
                 return  Page_IsValid;   
            }   
             else  {   
                 if  ( ! theForm.onsubmit  ||  (theForm.onsubmit()  !=   false )) {   
                    theForm.__EVENTTARGET.value  =  eventTarget;   
                    theForm.__EVENTARGUMENT.value  =  eventArgument;   
                    theForm.submit();   
                }   
                 return   true ;   
            }   
  
  
        }   
     </ script >   
    < asp:TextBox  ID ="DiscountRate"  runat ="server"  Style ="float: none; width: 50px;"   
    OnTextChanged ="UpdateDiscountedCourseFeeOnTextChanged"  AutoPostBack ="true"  CausesValidation ="true"   /> %      
                     < asp:RangeValidator  ID ="DiscountRateRangeValidator"  runat ="server"  ControlToValidate ="DiscountRate"      
                        MinimumValue ="0.00"  MaximumValue ="100.00"  Type ="Double"  ErrorMessage ="Discount Rate must be within 0-100"      
                        ToolTip ="Discount Rate must be within 0-100"  ValidationGroup ="PaymentGroup"  SetFocusOnError ="true"  Display ="dynamic" > * </ asp:RangeValidator >      
      </ form >        
</ body >   
</ html >