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<!DOCTYPE html>
<html>
<body>
<style>
html {
background: red;
transition: all 0.1s;
}
</style>
<script>
window.addEventListener("message", () => {
document.documentElement.style.background = "green";
});
document.documentElement.addEventListener(
"transitionend", () => {
window.parent.postMessage("complete", "*");
}
);
</script>
</body>
</html>