header {
  z-index: 1;
  position: sticky;
  position: -webkit-sticky;
  top: 0;
  box-sizing: border-box;
  margin: 0;
  padding: 0.4em;
  background-color: #222;
}

h2:target,
h3:target,
h4:target,
h5:target,
h6:target {
  /* sticky header に合わせて link scroll をずらす */
  margin-top: calc(-50px - 0.2em);
  padding-top: calc(50px + 0.2em);
}

header ul {
  display: flex;
  flex-direction: row;
  align-items: stretch;
  width: 80%;
  margin: 0 auto;
  padding: 4px;
}

header ul li {
  padding: 0;
  margin: 0;
  width: 30px;
  height: 30px;
  margin-left: 1em;
}

header ul li a {
  width: 30px;
  height: 30px;
  display: block;
}

header ul li svg {
  fill: #fff;
}

header ul li::before {
  content: none;
}

header ul li:first-child {
  flex-grow: 1;
  margin-left: 0;
}

header .logo {
  border: 1px solid #ccc;
  box-sizing: border-box;
}