diff -r 5a3c83412f79 -r f9078c6caeb6 mercurial/hgweb/hgweb_mod.py
--- a/mercurial/hgweb/hgweb_mod.py	Thu Mar 08 16:43:32 2018 -0800
+++ b/mercurial/hgweb/hgweb_mod.py	Thu Mar 08 16:22:25 2018 -0800
@@ -351,7 +351,7 @@
                 if args:
                     wsgireq.form['file'] = args
 
-            ua = wsgireq.env.get('HTTP_USER_AGENT', '')
+            ua = req.headers.get('User-Agent', '')
             if cmd == 'rev' and 'mercurial' in ua:
                 wsgireq.form['style'] = ['raw']