insertion-in-first-line-10.html

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<!DOCTYPE html>

<style>

  div { color: red; }

  div::first-line { color: green }

  #y { display: inline-block; }

</style>

<div id="x"><span id="y">This should be </span></div>

<script>

  x.offsetWidth;

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

  span.textContent = 'green';

  y.appendChild(span);

</script>