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                p 關鍵字 - 靠左,靠上
p 數值 - 由左向右偏移20px,由上向下偏移40px
p 百分比(1~100%) - 由左向右偏移20%,由上向下偏移40%



  width: 400px
  height: 200px
  border: 1px solid #aaa //框線,用以確認範圍
  background-image: url("")
  background-repeat: no-repeat // 背景圖片不重複,用以確認效果
    background-position: left top
    background-position: 20px 40px
    background-position: 20% 40%