<!doctype html>
<html>
<head>
<script type="text/javascript">
setInterval(function() {
var r = document.getElementById('r');
r.style.strokeWidth = 10 * (1 + Math.sin(+new Date() / 250));
}, 10);
</script>
</head>
<body>
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<rect width="100" height="100" stroke="red" id="r" />
</g>
</svg>
</body>
</html>