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

body {
  height: 100vh;
  width: 100%;
  border: 1px solid red;
}

.square {
  position: absolute;
  width: 200px;
  height: 200px;
  background-color: pink;
  bottom: 0;
  left: 0;
}