diff --git a/layout/base/tests/chrome/chrome.ini b/layout/base/tests/chrome/chrome.ini
index abeb20a06c39..fbc4204f2b60 100644
--- a/layout/base/tests/chrome/chrome.ini
+++ b/layout/base/tests/chrome/chrome.ini
@@ -67,8 +67,8 @@ skip-if = webrender
 [test_getClientRectsAndTexts.html]
 [test_css_visibility_propagation.xhtml]
 support-files =
-  window_css_visibility_propagation-1.html
-  window_css_visibility_propagation-2.html
+  window_css_visibility_propagation-1.xhtml
+  window_css_visibility_propagation-2.xhtml
   window_css_visibility_propagation-3.html
   window_css_visibility_propagation-4.html
   frame_css_visibility_propagation.html
diff --git a/layout/base/tests/chrome/test_css_visibility_propagation.xhtml b/layout/base/tests/chrome/test_css_visibility_propagation.xhtml
index bfe36041326f..02dbf34a8e38 100644
--- a/layout/base/tests/chrome/test_css_visibility_propagation.xhtml
+++ b/layout/base/tests/chrome/test_css_visibility_propagation.xhtml
@@ -13,10 +13,10 @@ const baseURL = "chrome://mochitests/content/chrome/layout/base/tests/chrome/";
 add_task(async () => {
   // Open two new windows to swap iframes.
   const window1 = docShell.rootTreeItem.domWindow.open(
-      baseURL + "window_css_visibility_propagation-1.html",
+      baseURL + "window_css_visibility_propagation-1.xhtml",
       "_blank", "chrome");
   const window2 = docShell.rootTreeItem.domWindow.open(
-      baseURL + "window_css_visibility_propagation-2.html",
+      baseURL + "window_css_visibility_propagation-2.xhtml",
       "_blank", "chrome");
 
   const loadWindow1 =
diff --git a/layout/base/tests/chrome/window_css_visibility_propagation-1.html b/layout/base/tests/chrome/window_css_visibility_propagation-1.html
deleted file mode 100644
index c1021968120d..000000000000
--- a/layout/base/tests/chrome/window_css_visibility_propagation-1.html
+++ /dev/null
@@ -1,3 +0,0 @@
-<div id="parent" style="visibility: hidden">
-  <iframe src="about:mozilla"/>
-</div>
diff --git a/layout/base/tests/chrome/window_css_visibility_propagation-1.xhtml b/layout/base/tests/chrome/window_css_visibility_propagation-1.xhtml
new file mode 100644
index 000000000000..1d8bd0f4cac7
--- /dev/null
+++ b/layout/base/tests/chrome/window_css_visibility_propagation-1.xhtml
@@ -0,0 +1,6 @@
+<?xml version="1.0"?>
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<div id="parent" style="visibility:hidden">
+  <iframe type="content" src="about:mozilla"></iframe>
+</div>
+</window>
diff --git a/layout/base/tests/chrome/window_css_visibility_propagation-2.html b/layout/base/tests/chrome/window_css_visibility_propagation-2.html
deleted file mode 100644
index 7ebdd9a6aaa3..000000000000
--- a/layout/base/tests/chrome/window_css_visibility_propagation-2.html
+++ /dev/null
@@ -1,3 +0,0 @@
-<div id="parent">
-  <iframe src="frame_css_visibility_propagation.html"/>
-</div>
diff --git a/layout/base/tests/chrome/window_css_visibility_propagation-2.xhtml b/layout/base/tests/chrome/window_css_visibility_propagation-2.xhtml
new file mode 100644
index 000000000000..64258a08116a
--- /dev/null
+++ b/layout/base/tests/chrome/window_css_visibility_propagation-2.xhtml
@@ -0,0 +1,6 @@
+<?xml version="1.0"?>
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<div id="parent">
+  <iframe type="content" src="frame_css_visibility_propagation.html"></iframe>
+</div>
+</window>
diff --git a/layout/base/tests/chrome/window_css_visibility_propagation-3.html b/layout/base/tests/chrome/window_css_visibility_propagation-3.html
index b2c4018831e0..91a2230ee107 100644
--- a/layout/base/tests/chrome/window_css_visibility_propagation-3.html
+++ b/layout/base/tests/chrome/window_css_visibility_propagation-3.html
@@ -1,3 +1,3 @@
 <div id="parent">
-  <iframe onload="opener.postMessage('ready');" src="window_css_visibility_propagation-2.html"/>
+  <iframe onload="opener.postMessage('ready');" src="window_css_visibility_propagation-2.xhtml"/>
 </div>