main.css

header {
  height: 100px;
  background: black;
  color: white;
  padding: 4px;
}

h1 {
  font-family: 'Helvetica', 'Arial', sans-serif;
  font-size: 64px;
  text-align: center;
  margin: 0;
}

main {
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  width: 100vw;
  height: 100vh;
  justify-content: space-evenly;
  align-items: space-evenly;
}

section {
  width: 40vw;
  height: 40vh;
  margin: 16px;
  display: flex;
  justify-content: center;
  align-items: center;
}

#square {
  width: 40vw;
  height: 40vw;
}

#circle {
  width: 40vw;
  height: 40vw;
  border-radius: 20vw;
}

#rectangle {
  width: 40vmax;
  height: 20vmax;
  margin-top: 64px;
}

#triangle {
  width: 0; 
  height: 0; 
  border-left: 40vmin solid transparent;
  border-right: 40vmin solid transparent;
  border-bottom: 60vmin solid pink;
  border-top: none;
  padding-top: 30vmin;
}

.yellow {
  background: yellow;
}

.red {
  background: red;
}

.blue {
  background: blue;
  color: white;
}

.bigBorders {
  border: 15px solid black;
}