<style>
:matches(foobar, .inlineContainer .floatContainer)::first-letter {
 font-size: 10px;
}
</style>

<span class=inlineContainer><div style="float: left" class=floatContainer>Pass if no crash</div></span>
<script>
document.body.offsetLeft;
document.querySelector(".floatContainer").style.marginRight = "10px";
if (window.testRunner)
    testRunner.dumpAsText();
</script>
