firefox/testing/web-platform/tests/css/css-ruby/pseudo-first-line.html

<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-ruby/#placement">
<link rel="help" href="https://drafts.csswg.org/css-pseudo/#first-line-styling">
<link rel="match" href="pseudo-first-line-ref.html">
<style>
div::first-line {
  ruby-position: under;
  color: orange;
}
ruby::first-line, rt::first-line {
  color: red;
}
</style>
<div>foo <ruby>base<rt>annotation</rt></ruby><br>
bar <ruby>base<rt>annotation</rt></ruby></div>