Yes, you need both. Here's a template:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
</head>
<body>
<h2><a name="debriefing:end1"></a><br>
End #1
</h2><br><p>
End #1 debriefing screen text.
</p><br><hr>
<h2><a name="debriefing:end2"></a><br>
End #2
</h2><br><p>
End #2 debriefing screen text.
</p><br><hr>
<h2><a name="debriefing:end3"></a><br>
End #3
</h2><br><p>
End #3 debriefing screen text.
</p><br><hr>
<h2><a name="debriefing:end4"></a><br>
End #4
</h2><br><p>
End #4 debriefing screen text.
</p><br><hr>
</body>
</html>
The reason there's lack of info on this is because... Well... Most mission makers nowadays are MP only for some reason, and in MP it always shows the briefing screen before you start the mission regardless of whether or not there's a briefing.html file present in the mission's folder.
Heck, I bet you could ask today's mission making community if they knew you needed a briefing.html file to show a briefing before the mission begins when working in SP, and 90% of them wouldn't know it.