<svg>
	<mask id="mask">
		<circle r="10" cx="100" cy="100"
					 class="c-mask-circle"/>
	</mask>
  
  <text y="20" x="5">mask:</text>
  <circle r="10" cx="100" cy="100"
					 class="c-mask-circle"/>
</svg>
  
<svg>  
  <g mask="url(#mask)">
  	<image xlink:href="https://source.unsplash.com/500x500/?blossom" width="100%" height="100%"/>
  </g>
  <text y="20" x="5">Result:</text>
</svg>
BODY {
  background: #333;
  text-align: center;
}
svg {
  display: inline-block;
  width: 200px;
  height: 200px;
  margin: 2em 1em;
  border: 1px dashed yellowgreen;
}
.c-mask-circle {
  fill: white;
  stroke: white;
  stroke-width: 1;
  animation: stroke-width 5s infinite;
}

@keyframes stroke-width {
  50% {
    stroke-width: 170;
  }
}

text {
  font-size: 1.3em;
  fill: #FFF;
}
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