body {
  font-family: Arial, sans-serif;
  margin: 0;
  padding: 0;
}

header {
  background: #222;
  color: white;
  padding: 1rem;
}

nav {
  background: #eee;
  padding: 0.5rem;
}

nav button {
  margin-right: 0.5rem;
}

main {
  padding: 1rem;
}

.tab {
  display: none;
}

.tab.active {
  display: block;
}