<!doctype html>
<script src="../../resources/ahem.js"></script>
<style>
.sample {
    border: solid 1px; green;
    font: 20px monospace;
    padding: 5px;
    white-space: pre-wrap;
}
</style>
<div class="sample" id="target"> bc</div>
<br>
<div class="sample">XYZ bc</div>
<script>
const target = document.getElementById('target');
const text = target.firstChild;
document.body.offsetHeight;
text.insertData(0, 'XYZ');
</script>