You can open the pop-up window with your alert message:
onClick="validation()"
. . .
function validation() {
//validation process
//
popup = window.open('','','width=200,height=150,left=300,top=100');
popup.document.open();
popup.document.write('<p>Alert message</p>');
popup.document.write('<input type=button value=" OK " onclick="window.close()">');
popup.document.close();
}