<!doctype html>
<style>
fieldset::first-letter {
  border: inherit;
}
</style>
<fieldset style="border: 10px solid red;">
  This text should have a green border on the first letter.
</fieldset>
<script>
document.body.offsetTop;
document.querySelector('fieldset').style.borderColor = "green";
document.body.offsetTop;
</script>