function validate() {
  var form = this;
  var isValid = true;
  var mailre = /^[a-zA-Z0-9._%-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$/;
  var handle = document.getElementById('error');
  handle.innerHTML = "";

  if (form.elements['name'].value == "") {
    form.elements['name'].style['border'] = "2px solid #FF0000";
    isValid = false;
    handle.innerHTML += "<li>Podaj proszę imię.</li>";
  } else {
    form.elements['name'].removeAttribute('style');
  }

  if (form.elements['mail'].value == "") {
    form.elements['mail'].style['border'] = "2px solid #FF0000";
    isValid = false;
    handle.innerHTML += "<li>Podaj proszę swój email.</li>";
  } else if (!mailre.test(form.elements['mail'].value)) {
    form.elements['mail'].style['border'] = "2px solid #FF0000";
    isValid = false;
    handle.innerHTML += "<li>Email który podałeś jest nieprawidłowy.</li>";
  } else {
    form.elements['mail'].removeAttribute('style');
  }



  if(!isValid) {

  }
  return isValid;
}

function onLoad() {
  document.getElementById('forma').onsubmit = validate;
}

window.addEventListener("load", onLoad, false);