diff --git a/pyguide.html b/pyguide.html
index 704f980..026e61b 100644
--- a/pyguide.html
+++ b/pyguide.html
@@ -1,3 +1,4 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/REC-html4/strict.dtd">
 <HTML xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:dcq="http://purl.org/dc/qualifiers/1.0/" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:fn="http://www.w3.org/2005/xpath-functions">
 <HEAD>
 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
