[Shokoladniu]

так что ли?

function feedback() {
	var yname=document.getElementById('posName').value;
	var yemail=document.getElementById('posEmail').value;
	var ycomment=document.getElementById('posText').value;
	if((yname =='') || (yemail=='')  || (ycomment=='')){ 
		document.getElementById("alert").innerHTML = "<p style='color:red'>Введите все поля</p>";
		else  {
    document.getElementById("alert").innerHTML = ""
  }
return false; 
	}