function checkform() {
  var send = 0;
  var nb = 0;
  //contact fixe fields
  if(document.form.elements['names']){
    if(check('names')) send++;
    nb ++;
  }
  if(document.form.elements['email']){
    if(check('email')) send++;
    nb ++;
  }
  if(document.form.elements['activity']){
    if(check('activity')) send++;
    nb ++;
  }
  if(document.form.elements['antispam']){
    if(check('antispam')) send++;
    nb ++;
  }
  //contact conditionnal fields
  if(document.form.elements['use']){
    if(check('use')) send++;
    nb ++;
  }
  if(document.form.elements['format']) {
    if(check('format')) send ++;
    nb ++;
  }
  if(document.form.elements['comment']) {
    if (check('comment')) send ++;
    nb ++;
  }

  //check for empty fields
  if(send>=nb){
    document.form.submit();
  }
  else {
    document.getElementById('check').style.display = 'block';
  }
}

function check(name){
  if(document.form.elements[name].value!='') {
    document.getElementById(name).removeAttribute('class');
    document.getElementById(name).removeAttribute('className');
    return true;
  }
  else {
    document.getElementById(name).setAttribute('class', 'marked');
    document.getElementById(name).setAttribute('className', 'marked');
    return false;
  }
}