遮罩內容單位

html,
body,
svg {
  height: 100%;
}

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <mask id="myMask1" maskContentUnits="userSpaceOnUse">
    <rect fill="black" x="0" y="0" width="100%" height="100%" />
    <circle fill="white" cx="50" cy="50" r="35" />
  </mask>

  <mask id="myMask2" maskContentUnits="objectBoundingBox">
    <rect fill="black" x="0" y="0" width="100%" height="100%" />
    <circle fill="white" cx=".5" cy=".5" r=".35" />
  </mask>

  <!-- Some reference rect to materialized the mask -->
  <rect id="r1" x="0" y="0" width="45" height="45" />
  <rect id="r2" x="0" y="55" width="45" height="45" />
  <rect id="r3" x="55" y="55" width="45" height="45" />
  <rect id="r4" x="55" y="0" width="45" height="45" />

  <!-- The first 3 rect are masked with useSpaceOnUse units -->
  <use mask="url(#myMask1)" href="#r1" fill="red" />
  <use mask="url(#myMask1)" href="#r2" fill="red" />
  <use mask="url(#myMask1)" href="#r3" fill="red" />

  <!-- The last rect is masked with objectBoundingBox units -->
  <use mask="url(#myMask2)" href="#r4" fill="red" />
</svg>