svg蒙版mask

<svg width="400" height="300">
  <defs>
      <mask id="small-rect">
          <rect x="0" y="0" width="400" height="300" fill="white"></rect>
          <rect width="100" height="100" fill="black" x="200" y="100"></rect>
      </mask>
  </defs>
  <rect id="back" x="0" y="0" width="400" height="300" fill="#d4fcff"></rect>
  <rect id="front" x="0" y="0" width="400" height="300" fill="#fcd3db" mask="url(#small-rect)"></rect>
</svg>