The following is a simplified description of the process that happens when a validation control is used on a page.
1. When a page containing validation controls is requested, the user's browser type and version is evaluated to determine if client-side validation routines should be used.
2. If the browser supports client-side validation, the page loads with the appropriate routines being called from the validation controls. This is handled through a JScript® include file.
3. When the user moves between controls on a client-side validated form after changing a control's value, the validation events for the control that lost the focus are fired and appropriate error messages (if any) are displayed.
4. If the user generates a form submit on a client-side validated form, the entire form is evaluated for any validation controls that are not valid. If even one control is not valid, the form will not be submitted.
5. When the form is posted back to the server through a postback event, it is evaluated for validity on the server. This occurs even if the form was already evaluated on the client.
6. If any of the validation controls are found to be invalid through the server-side check, the page is redisplayed with the appropriate error messages included.
7. If the form passes all validation on both the server and client, the page processing sequence continues with the event that triggered the postback firing.
Disabling Client-Side Validation on the Entire Form

An easy way to disable all client-side form validation is to provide the clienttarget=downlevel attribute in the Page directive at the top of your page. The directive looks like this:
<%@ Page Language="VB" codebehind="MyPage1.vb" Inherits
MyApp.MyPage1" clienttarget=downlevel>

You need to be careful when you do this because it not only disables all client-side validation, it also disables any other client-side server control functionality. You can also disable client-side form validation for all validation controls by setting the global page variable Page_PostIfValidationError to True.
The only time you would do either of these is when you have included some validation controls through a common include for the page or embedded them in a custom control. Alternately, you can opt not to include the validation controls on the page.
You can disable all client-side form validation using a number of different methods depending upon the type of control that the user activates. For a regular HTML control, you can set the Page_ValidationActive property to false. For example:
<input type=button runat=server id=cmdCancel value="Cancel"
onClick="Page_ValidationActive=false;"
onServerClick="cmdCancel_Click">
Please note that it is important to set the onServerClick event to equal the event you have defined in the codebehind page for the control; otherwise, the event won't fire since you have overridden the onClick event.
For a server-side control, disabling all form validation requires some client-side code, like this:
<asp:button runat=server id=cmdCancel Text="Cancel"
onClick="cmdCancel_Click">

<script language="JavaScript>
document.all["cmdCancel"].onclick =
new function("Page_ValidationActive=false;");
</script>
The effect of these last two examples is that all client-side validation has been turned off when the Cancel button is pressed. You will still have to bypass server-side validation if the server code uses the Page.IsValid property