insertion-in-first-line-8.html

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<!DOCTYPE html>

<style>

  div { color: red; }

  div::first-line { color: green }

</style>

<div id="x"><span id="y">green</span></div>

<script>

  x.offsetWidth;

  var span = document.createElement('span');

  span.textContent = 'This should be ';

  y.insertBefore(span, y.firstChild);

</script>