* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: sans-serif;
}

header {
  height: 100vh;
  background-image: url("https://images.pexels.com/photos/1164763/pexels-photo-1164763.jpeg?auto=compress&cs=tinysrgb&dpr=2&h=650&w=940");
  background-size: cover;
}

nav ul {
  display: flex;
  justify-content: space-around;
  align-items: center;
  height: 10vh;
  position: fixed;
  top: 0;
  left: 0;
  background-color: white;
  width: 100%;
  list-style: none;
}

#one,
#two,
#three {
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  color: white;
}

#one {
  background-color: rgb(43, 87, 102);
}

#two {
  background-color: rgb(56, 43, 102);
}

#three {
  background-color: rgb(102, 81, 43);
}

nav a {
  color: black;
  font-size: 18px;
  text-decoration: none;
  position: relative;
}

nav a::after {
  content: "";
  width: 100%;
  height: 100%;
  position: absolute;
  border-bottom: 2px solid green;
  top: 0;
  left: 0;
  transform: translateX(-100%);
  transition: transform 0.5s ease;
}

nav li {
  overflow: hidden;
  height: 25px;
}

a.active::after {
  transform: translateX(0%);
}