Margin-Bottom of #header li doesnt work :/

Question

  html, body {
      height: 100%;
      width: 99.9%;
  }

  #header {
      background-image: url(../img/img-back.jpg);
      border: 1px solid black;
      width: 80%;
      height: 90px;
      margin: 0 auto;
  }
  #body-block {
      background-image: url(../img/img-back.jpg);
      border: 1px solid black;
      height: 80%;
      width: 80%;
      margin: 0 auto;
  }
  #footer {
      background-image: url(../img/img-back.jpg);
      border: 1px solid black;
      height: 11%;
      width: 80%;
      margin: 0 auto;
  }
  #img-icon {
      height: 80px;
      width: 80px;
  }
  #navbar-block{
      height: 80%;
      width: 80%;
  }
  #header li{
    overflow: hidden;
    float:left;
    padding-left: 50px;
    font: 20px sans-serif;
    margin-left: 40px;
    margin-bottom: 30px; //-- here is the mentioned style 
}

can drop HTML file also if needed