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chromium/third_party/blink/web_tests/editing/inserting/typing_letter_spacing.html 

<!doctype html>
<script src="../../resources/ahem.js"></script>
<style>
#target {
    font: 10px/15px Ahem;
    letter-spacing: 20px;
}
</style>
<div id="target">x</div>
<script>
const text = target.firstChild;
function typing(character) {
    text.replaceData(0, 0, character);
    document.body.offsetHeight;
}
typing('a');
typing('b');
typing('c');
</script>