html,
body {
  height: 100%;
  width: 100%;
  padding: 0;
  margin: 0;
}
* {
  box-sizing: border-box;
}

.paddle {
  position: fixed;
  /* fixed or absolute? */
  background-color: lightseagreen;
}

.paddle-horizontal {
  width: 100px;
  height: 20px;
}

.paddle-vertical {
  width: 20px;
  height: 100px;
}

.paddle-top {
  top: 0;
}

.paddle-bottom {
  bottom: 0;
}

.paddle-left {
  left: 0;
}

.paddle-right {
  right: 0;
}

.ball {
  position: fixed;
  height: 25px;
  width: 25px;
  background-color: purple;
  border-radius: 50%;
  display: inline-block;
  /* Starting position */
  top: 123px;
  left: 123px;
}

.container {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100%;
}