*,
*:before,
*:after {
  box-sizing: border-box;
}

body {
  background-color: lightgray;
  line-height: 1.4;
}

header {
  padding: 30px;
  color: white;
  width: 100%;
  background-color: black;
  flex: 35%;
  margin-bottom: 5%;
}
header p {
  max-width: 50%;
  font-size: 18px;
}
header p a {
  color: yellow;
  text-decoration: none;
}

section {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}
section .container {
  width: 100%;
  max-width: 80%;
  margin: 0 auto;
  flex: 60%;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 80px 0;
}
section .container ul {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  width: 100%;
  position: relative;
  padding: 0 0 56.25%;
}
section .container ul li {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 0;
  padding: 0 0 56.25%;
}
section .container ul li h2 {
  background-color: black;
  color: white;
  position: absolute;
  height: 80px;
  top: -80px;
  display: flex;
  justify-content: center;
  align-items: center;
  margin: 0;
  width: calc(100% / 3);
  border-left: 1px solid lightgray;
  font-size: 18px;
  transition: background-color 200ms ease-in;
}
section .container ul li:first-child h2 {
  border-left: none;
}
section .container ul li.active {
  z-index: 10;
}
section .container ul li.active h2 {
  background-color: blue;
}
section .container ul li iframe {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}