html,
body {
  overflow: hidden;
}

body {
  display: flex;
  flex-direction: column;
  min-height: 100vh;
  margin: 0;
  background-color: aqua;
}

header,
footer {
  height: 2rem;
  background-color: black;
}

main {
  display: flex;
  flex: 1;
  justify-content: center;
  align-items: center;
}

.starting-button,
.hello-button {
  min-width: 4rem;
  min-height: 2rem;
  border: 1px solid black;
  border-radius: .375rem;
  font-size: 1rem;
  background-color: yellow;
}

.hello-button {
  position: absolute;
}