header {
  z-index: 1;
  position: sticky;
  top: 0;width:100%;
  background: white;
  border-bottom: 1px solid lightgrey;
}
header nav ul{
  list-style: none;
  margin:0;
  padding:0;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
}
header nav ul li{
  flex-grow: 1;
  text-align: center;
  padding: 0 0.5em 0 0;
}
header nav ul li a{
  width:100%;
  padding:0.25em !important;
  margin:0.25em !important;
  display: block;
}