td, th {
  background-color: #F4F4F4;
}

thead tr {
    height: 0;
}

div.a {
    color: white;
}

thead:before {
    display: block;
    content: '';
    width: 100%;
    background: red;
    height: 20px;
    position: fixed;
    background: dodgerblue;
}

/* 999th try */
.a {
    position: fixed;
}