Event.observe(window, 'load', init, false);

function init(){
     Event.observe('webpage_url', 'keyup', validate, false);
}


function validate(){
     var url = 'validate.php';
     var pars = 'webpage_url='+escape($F('webpage_url'));
     var target = 'validate';
     var myAjax = new Ajax.Updater(target, url, {method: 'get', parameters: pars});
}