maskContentUnits

The maskContentUnits attribute indicates which coordinate system to use for the contents of the <mask> element.

Only one element is using this attribute: <mask>

<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)" xlink:href="#r1" fill="red" />
  <use mask="url(#myMask1)" xlink:href="#r2" fill="red" />
  <use mask="url(#myMask1)" xlink:href="#r3" fill="red" />

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

mask

For <mask>, maskContentUnits defines the coordinate system in use for the content of the element.

userSpaceOnUse

This value indicates that all coordinates inside the <mask> element refer to the user coordinate system as defined when the mask was created.

objectBoundingBox

This value indicates that all coordinates inside the <mask> element are relative to the bounding box of the element the mask is applied to. A bounding box could be considered the same as if the content of the <mask> were bound to a "0 0 1 1" viewbox.

Specifications