<svg>
	<mask id="mask">
		<circle r="10" cx="100" cy="100"
					 class="c-mask-circle"/>
	</mask>
  
  <g mask="url(#mask)">
  	<image xlink:href="https://source.unsplash.com/500x500/?blossom" width="100%" height="100%"/>
  </g>
</svg>
svg {
  display: block;
  width: 200px;
  height: 200px;
  margin: 2em auto;
  border: 1px solid #DDD;
}

.c-mask-circle {
  fill: white;
  animation: stroke-width 5s infinite;
}

@keyframes stroke-width {
  50% {
    transform: scale(10);
  }
}
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