[nerv_]

<svg style="width: 200px; height: 200px;">
    <rect id="rect" width="100" height="100" fill="lightsalmon"></rect>
</svg>

<script>
    var factor = 50;

    var id = setInterval(function() {
        rect.setAttribute('rx', factor);
        rect.setAttribute('ry', factor);
        factor--;
        if (!factor) clearInterval(id);
    }, 100);
</script>