As per current web development all web application are become Ajax base means get and display data on demand. So asp.net post back method not allowed using at more. I have same case with my current project. Which contain so many teleric control dev express. Get and display data after button click go to server side and take action. But client want to change this page from use Ajax request when server request required. Or use rest WCF service for fill all method.
Page already use regular expression validation, range validation, validation summery group
Now I don’t want to rewrite all page from scratch. So I start to modified from validation
Page_ClientValidate('') is good method on JavaScript side it will do all same action when we fire post method. Check all validation and do same task which it will do for post method.

So concussion is that you can use asp.net basic validation and custom validation control still when you use ajax call.

"Good to use Page_ClientValidate for validation"