<style>
test { transition:0.1s; display:block; height:100px; border: 2px solid green; }
.animate { display:inline-block; height:10px; width:10px; }
</style>
<test></test>
<script>
var test = document.querySelector("test");

if (window.testRunner) {
    testRunner.waitUntilDone();
    test.ontransitionend = () => testRunner.notifyDone();
}
test.offsetWidth;
test.className = "animate";
</script>