css/css-pseudo/first-letter-background-image-dynamic.html

<!doctype html>
<title>CSS Test: ::first-letter correctly applies background-image dynamically</title>
<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
<link rel="author" href="https://mozilla.org" title="Mozilla">
<link rel="help" href="https://drafts.csswg.org/css-pseudo-4/#first-letter-styling">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1528451">
<link rel="match" href="first-letter-background-image-ref.html">
<style>
  div::first-letter {
    color: lime;
  }
  div.image::first-letter {
    /* Lime background */
    background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M/wHwAEBgIApD5fRAAAAABJRU5ErkJggg==');
  }
</style>
<div>
  A letter
</div>
<script>
  let div = document.querySelector("div");
  getComputedStyle(div).color;
  div.classList.add('image');
</script>