.navigation {
   /* fixed keyword is fine too */
   position: sticky;
   top: 0;
   z-index: 100;
   /* z-index works pretty much like a layer:
   the higher the z-index value, the greater
   it will allow the navigation tag to stay on top
   of other tags */
}

nav {
    position: sticky; top: 0;
}

.fixed-content {
    top: 0;
    bottom:0;
    position:fixed;
    overflow-y:scroll;
    overflow-x:hidden;
}

#header {
  position: fixed;
}

#content {
  margin-top: 100px;
}