diff --git a/tests/s5.basic.html b/tests/s5.basic.html
index bcee42175..9f9b3e00e 100644
--- a/tests/s5.basic.html
+++ b/tests/s5.basic.html
@@ -1,5 +1,5 @@
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
-<html
+<html xmlns="http://www.w3.org/1999/xhtml"
 ><head
   ><title
     >My S5 Document</title
diff --git a/tests/s5.fancy.html b/tests/s5.fancy.html
index b929a1422..5e6e1b243 100644
--- a/tests/s5.fancy.html
+++ b/tests/s5.fancy.html
@@ -1,5 +1,5 @@
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
-<html
+<html xmlns="http://www.w3.org/1999/xhtml"
 ><head
   ><title
     >My S5 Document</title
diff --git a/tests/s5.inserts.html b/tests/s5.inserts.html
index 27b1ab7a9..2938dfcfe 100644
--- a/tests/s5.inserts.html
+++ b/tests/s5.inserts.html
@@ -1,5 +1,5 @@
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
-<html
+<html xmlns="http://www.w3.org/1999/xhtml"
 ><head
   ><title
     >My S5 Document</title
diff --git a/tests/writer.html b/tests/writer.html
index e674170da..9a6f7cecc 100644
--- a/tests/writer.html
+++ b/tests/writer.html
@@ -1,5 +1,5 @@
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
-<html
+<html xmlns="http://www.w3.org/1999/xhtml"
 ><head
   ><title
     >Pandoc Test Suite</title
