[NastyaNewer]

function validateForm() {
  var form = document.contact_form; // по имени
  var elements = form.querySelectorAll('.form_control'); 
  form.addEventListener('submit', validForm, false);
  function validForm(event) {
    var valid = true,
    border = '';
    if (form.username.value == '') {
      border = '2px solid red';
      valid = false;
    }
    form.username.style.border = border;
    border = '';
    if (form.email.value == '') {
      border = '2px solid red';
      valid = false;
    }
    form.email.style.border = border;
    if (!valid) {
      event.preventDefault();
    }
  }
}