<svg id="svg-root"
  width="200" height="200"
  xmlns="http://www.w3.org/2000/svg"
  xmlns:html="http://www.w3.org/1999/xhtml">
  <g id="testmeta">
    <title>filter applied to element in mask, clipped by filter region</title>
    <html:link rel="help" href="https://www.w3.org/TR/css-masking-1"/>
    <html:link rel="match" href="reference/mask-green-square-001-ref.svg" />
  </g>
  <defs>
    <filter id="filter" x="0" y="0" width="1" height="1">
      <feGaussianBlur stdDeviation="0.001"/>
    </filter>

    <mask id="maskFill" maskContentUnits="userSpaceOnUse">
      <rect x="25" y="25" width="50" height="50" stroke-width="50" fill="none"
            stroke="white" filter="url(#filter)"/>
      <rect x="25" y="25" width="50" height="50" fill="black"/>
    </mask>
  </defs>

  <g transform="translate(50, 50)">
    <rect width="100" height="100" fill="green" />
    <rect width="100" height="100" mask="url(#maskFill)" fill="red"/>
  </g>
</svg>