<style>
.c { background-color: red; color: yellow; }
.t { background-color: inherit }
.fastpath-mutation { background-color: green; color: blue; }
</style>
<div id=c class=c><div id=t class=t>Text</div></div>
<script>
document.body.offsetLeft;
c.classList.add("fastpath-mutation");
</script>