third_party/blink/web_tests/shadow-dom/host-pseudo-elements.html - manifest_repos/chromium_src - Git at Google

 <!DOCTYPE html>
 <style>
   #host::after {
     content: 'After';
   }
 </style>
 <div id="host">Host</div>
 <script>
 host.attachShadow({ mode: 'open'}).innerHTML = '<span><slot></slot> ';
 </script>
	<!DOCTYPE html>
	<style>
	#host::after {
	content: 'After';
	}
	</style>
	<div id="host">Host</div>
	<script>
	host.attachShadow({ mode: 'open'}).innerHTML = '<span><slot></slot> ';
	</script>