--- a/data/doc/manual.docbook	2016-11-05 09:22:50.000000000 +0100
+++ b/data/doc/manual.docbook	2017-07-28 18:03:51.938394459 +0200
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?> <!-- -*- mode:nxml; indent-tabs-mode:nil; fill-column:80; -*- -->
-  <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.0//EN"
-                      "http://www.oasis-open.org/docbook/xml/4.0/docbookx.dtd">
+  <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.2//EN"
+                      "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
 <book id="Hydrogen-manual" lang="en">
 
   <bookinfo>
--- a/data/doc/tutorial.docbook	2016-11-05 09:22:50.000000000 +0100
+++ b/data/doc/tutorial.docbook	2017-07-28 18:03:51.938394459 +0200
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.0//EN"
-               "http://www.oasis-open.org/docbook/xml/4.0/docbookx.dtd">
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.2//EN"
+               "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
 <book id="Hydrogen-tutorial" lang="en">
 
 <bookinfo>