index.css

body {
  display: flex;
  align-items: center;
  justify-content: center;
}
section {
  width: 488px;
  position: relative;
}
button {
  position: absolute;
  top: calc(45vh - 50px);
  left: 10%;
  width: 80%;
  height: 60px;
  background-color: transparent;
  border: 1px solid gainsboro;
  border-radius: 10px;
  padding: 10px;
}
button + button {
  top: calc(45vh + 30px);
}
input[type=file]{
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  opacity: 0;
  filter: alpha(opacity=0);
  cursor: pointer;
}
.myCanvas {
  position: absolute;
  top: calc(5vh);
  left: 10%;
  width: 80%;
  height: 250px;
  background-color: #f5f5f5;
  border-radius: 10px;
}
.myVideo {
  position: absolute;
  top: calc(60vh);
  left: 10%;
  width: 80%;
  height: 250px;
  background-color: #f5f5f5;
  border-radius: 10px;
}