Add extra HTTP headers for better standards-compliance

Change-Id: I5285ae491842b240c3d1da0b71b837df7bb1561f
diff --git a/visualisation/visualisation.py b/visualisation/visualisation.py
index cf8480a..3f3462b 100644
--- a/visualisation/visualisation.py
+++ b/visualisation/visualisation.py
@@ -130,8 +130,10 @@
             cache.last_update = datetime.datetime.utcnow()
 
         page = []
+        page.append('<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">')
         page.append('<html>')
         page.append('<head>')
+        page.append('<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">')
         page.append('<TITLE>VLANd visualisation</TITLE>')
         page.append('<link rel="stylesheet" type="text/css" href="style.css">')
         if config.refresh and config.refresh > 0: