body {
  font-family: Arial, sans-serif;
  background-color: #f5f5f5;
  margin: 20px;
  text-align: center;
}

/* Title */
h1 {
  margin-bottom: 10px;
}

p {
  margin-bottom: 30px;
}

#container {
  background-color: lightblue;
  width: 500px;
  height: 350px;
  margin: 0 auto;
  padding: 20px;
  position: relative;
  border: 2px solid #333;
}

.square,
.circle {
  width: 100px;
  height: 100px;
  display: inline-block;   /* 可改成 block / inline / none 试效果 */
  margin: 10px;
  position: relative;      /* 可改成 static / absolute / fixed */
  float: none;             /* 可改成 left / right */
}

.circle {
  border-radius: 50%;
}

#blockRed {
  background-color: red;
  top: 0;
  left: 0;
}

#blockYellow {
  background-color: yellow;
  top: 20px;
  left: 10px;
}

#circleRed {
  background-color: red;
  top: 30px;
  left: 20px;
}

#circleYellow {
  background-color: yellow;
  top: 10px;
  left: 30px;
}