function formLeeren()
{
  var form = document.forms["mailform"];
    for(i=0;i<form.elements.length;i++)
    {
        if(form.elements[i].type == 'text' || form.elements[i].type == 'textarea')
        {
            form.elements[i].onfocus = function()
              {
                if(this.value==this.defaultValue)
                  {
                    this.value='';
                  }
              }
            form.elements[i].onblur = function()
              {
                if(this.value=='')
                  {
                    this.value=this.defaultValue;
                  }
              }
        }
    }
}
window.onload=formLeeren;