<!DOCTYPE html>
<style>
#blur {
    position: absolute;
    width: 200px;
    height: 200px;
    left: 100px;
    top: 100px;

    box-sizing: border-box;
    border: 1px solid green;

    -webkit-backdrop-filter: blur(50px);
}
</style>
<div id="blur"></div>
<pre id="log"></pre>
<script>
    if (window.testRunner)
        testRunner.dumpAsText();
    const log = document.getElementById("log");
    if (window.internals)
        log.innerText = window.internals.platformLayerTreeAsText(document.getElementById("blur"));
</script>