diff --git a/layout/reftests/bugs/255820-1-ref.html b/layout/reftests/bugs/255820-1-ref.html
new file mode 100644
index 00000000000..98f550633d5
--- /dev/null
+++ b/layout/reftests/bugs/255820-1-ref.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta http-equiv="content-type" content="text/html; charset=us-ascii">
+</head>
+<body>
+  <iframe width="400" height="400" src="data:application/xhtml+xml;charset=us-ascii,<html xmlns='http://www.w3.org/1999/xhtml'><h1>Test</h1></html>"></iframe>
+</body>
+</html>
diff --git a/layout/reftests/bugs/255820-1.html b/layout/reftests/bugs/255820-1.html
new file mode 100644
index 00000000000..560083ae212
--- /dev/null
+++ b/layout/reftests/bugs/255820-1.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta http-equiv="content-type" content="text/html; charset=us-ascii">
+</head>
+<body>
+  <iframe width="400" height="400" src="data:application/xhtml+xml;charset=UTF-16LE,<%00h%00t%00m%00l%00 %00x%00m%00l%00n%00s%00=%00'%00h%00t%00t%00p%00:%00/%00/%00w%00w%00w%00.%00w%003%00.%00o%00r%00g%00/%001%009%009%009%00/%00x%00h%00t%00m%00l%00'%00>%00<%00h%001%00>%00T%00e%00s%00t%00<%00/%00h%001%00>%00<%00/%00h%00t%00m%00l%00>%00"></iframe>
+</body>
+</html>
diff --git a/layout/reftests/bugs/reftest.list b/layout/reftests/bugs/reftest.list
index 2dab5a03752..1cd9405ba20 100644
--- a/layout/reftests/bugs/reftest.list
+++ b/layout/reftests/bugs/reftest.list
@@ -83,6 +83,7 @@ fails == 25888-3r.html 25888-3r-ref.html # bug 25888
 == 244932-1.html 244932-1-ref.html
 == 249982-1.html 249982-1-ref.html
 == 253701-1.html 253701-1-ref.html
+== 255820-1.html 255820-1-ref.html
 == 262151-1.html 262151-1-ref.html
 == 267353-1.html 267353-1-ref.html
 == 273681-1.html 273681-1-ref.html
