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chromium/third_party/blink/web_tests/paint/selection/first_letter/first_letter_1rr.html 

<!doctype html>
<style>p::first-letter { font-size: 500%; color: red; }</style>
<p>  [!(5)#]9abcdefghi</p>
<script>
const selection = window.getSelection();
const sample = document.querySelector('p');
// Select "abc"
selection.collapse(sample.firstChild, 10);
selection.extend(sample.firstChild, 13);
</script>