#header {
    margin-bottom:2em;
}

#footer  ul {width :auto; display:inline;  }
#footer  ul li {width :auto; display: block}

body{ min-height:100vh; margin:0; position:relative; }
footer{  }

/* Trick: */
body {
  position: relative;
}

body::after {
  content: '';
  display: block;
  height: 12em; /* Set same as footer's height */
}

footer {
  position: absolute;
  bottom: 0;
  height: 12em;
  width:100%;
  max-width:1500px;
}