diff --git a/Lib/http/server.py b/Lib/http/server.py
--- a/Lib/http/server.py
+++ b/Lib/http/server.py
@@ -734,7 +734,8 @@
         list.sort(key=lambda a: a.lower())
         r = []
         displaypath = html.escape(urllib.parse.unquote(self.path))
-        r.append('<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">')
+        r.append('<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" '
+                 '"http://www.w3.org/TR/html4/strict.dtd">')
         r.append("<html>\n<title>Directory listing for %s</title>\n" % displaypath)
         r.append("<body>\n<h2>Directory listing for %s</h2>\n" % displaypath)
         r.append("<hr>\n<ul>\n")
