diff --git a/data/templates/default.html5 b/data/templates/default.html5
index 5641ecb80..7d1b834eb 100644
--- a/data/templates/default.html5
+++ b/data/templates/default.html5
@@ -1,17 +1,17 @@
 <!DOCTYPE html>
-<html$if(lang)$ lang="$lang$"$endif$$if(dir)$ dir="$dir$"$endif$>
+<html xmlns="http://www.w3.org/1999/xhtml" lang="$lang$" xml:lang="$lang$"$if(dir)$ dir="$dir$"$endif$>
 <head>
-  <meta charset="utf-8">
-  <meta name="generator" content="pandoc">
-  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
+  <meta charset="utf-8" />
+  <meta name="generator" content="pandoc" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
 $for(author-meta)$
-  <meta name="author" content="$author-meta$">
+  <meta name="author" content="$author-meta$" />
 $endfor$
 $if(date-meta)$
-  <meta name="dcterms.date" content="$date-meta$">
+  <meta name="dcterms.date" content="$date-meta$" />
 $endif$
 $if(keywords)$
-  <meta name="keywords" content="$for(keywords)$$keywords$$sep$, $endfor$">
+  <meta name="keywords" content="$for(keywords)$$keywords$$sep$, $endfor$" />
 $endif$
   <title>$if(title-prefix)$$title-prefix$ – $endif$$pagetitle$</title>
   <style type="text/css">code{white-space: pre;}</style>
diff --git a/test/lhs-test.html b/test/lhs-test.html
index 2c3b6b0f8..759b0955b 100644
--- a/test/lhs-test.html
+++ b/test/lhs-test.html
@@ -1,9 +1,9 @@
 <!DOCTYPE html>
-<html>
+<html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang="">
 <head>
-  <meta charset="utf-8">
-  <meta name="generator" content="pandoc">
-  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
+  <meta charset="utf-8" />
+  <meta name="generator" content="pandoc" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
   <title></title>
   <style type="text/css">code{white-space: pre;}</style>
   <style type="text/css">
diff --git a/test/lhs-test.html+lhs b/test/lhs-test.html+lhs
index 443b0642f..5fed70ddc 100644
--- a/test/lhs-test.html+lhs
+++ b/test/lhs-test.html+lhs
@@ -1,9 +1,9 @@
 <!DOCTYPE html>
-<html>
+<html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang="">
 <head>
-  <meta charset="utf-8">
-  <meta name="generator" content="pandoc">
-  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
+  <meta charset="utf-8" />
+  <meta name="generator" content="pandoc" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
   <title></title>
   <style type="text/css">code{white-space: pre;}</style>
   <style type="text/css">
diff --git a/test/writer.html5 b/test/writer.html5
index 8e0dff764..6a1e8388d 100644
--- a/test/writer.html5
+++ b/test/writer.html5
@@ -1,12 +1,12 @@
 <!DOCTYPE html>
-<html>
+<html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang="">
 <head>
-  <meta charset="utf-8">
-  <meta name="generator" content="pandoc">
-  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
-  <meta name="author" content="John MacFarlane">
-  <meta name="author" content="Anonymous">
-  <meta name="dcterms.date" content="2006-07-17">
+  <meta charset="utf-8" />
+  <meta name="generator" content="pandoc" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
+  <meta name="author" content="John MacFarlane" />
+  <meta name="author" content="Anonymous" />
+  <meta name="dcterms.date" content="2006-07-17" />
   <title>Pandoc Test Suite</title>
   <style type="text/css">code{white-space: pre;}</style>
   <!--[if lt IE 9]>
