diff --git a/testing/web-platform/tests/domparsing/XMLSerializer-serializeToString.html b/testing/web-platform/tests/domparsing/XMLSerializer-serializeToString.html
index d6e9182ad4b5..a4d6eff4bd4e 100644
--- a/testing/web-platform/tests/domparsing/XMLSerializer-serializeToString.html
+++ b/testing/web-platform/tests/domparsing/XMLSerializer-serializeToString.html
@@ -503,6 +503,165 @@
 (
 )
 {
+assert_equals
+(
+serialize
+(
+parse
+(
+'
+<
+root
+>
+<
+child
+xmlns
+=
+"
+"
+/
+>
+<
+/
+root
+>
+'
+)
+)
+'
+<
+root
+>
+<
+child
+/
+>
+<
+/
+root
+>
+'
+)
+;
+assert_equals
+(
+serialize
+(
+parse
+(
+'
+<
+root
+xmlns
+=
+"
+"
+>
+<
+child
+xmlns
+=
+"
+"
+/
+>
+<
+/
+root
+>
+'
+)
+)
+'
+<
+root
+>
+<
+child
+/
+>
+<
+/
+root
+>
+'
+)
+;
+assert_equals
+(
+serialize
+(
+parse
+(
+'
+<
+root
+xmlns
+=
+"
+u1
+"
+>
+<
+child
+xmlns
+=
+"
+u1
+"
+/
+>
+<
+/
+root
+>
+'
+)
+)
+'
+<
+root
+xmlns
+=
+"
+u1
+"
+>
+<
+child
+/
+>
+<
+/
+root
+>
+'
+)
+;
+}
+'
+Check
+if
+redundant
+xmlns
+=
+"
+.
+.
+.
+"
+is
+dropped
+.
+'
+)
+;
+test
+(
+function
+(
+)
+{
 const
 root
 =
@@ -2099,97 +2258,6 @@
 '
 <
 root
->
-<
-child
-xmlns
-=
-"
-"
-/
->
-<
-/
-root
->
-'
-)
-)
-'
-<
-root
->
-<
-child
-/
->
-<
-/
-root
->
-'
-)
-;
-assert_equals
-(
-serialize
-(
-parse
-(
-'
-<
-root
-xmlns
-=
-"
-u1
-"
->
-<
-child
-xmlns
-=
-"
-u1
-"
-/
->
-<
-/
-root
->
-'
-)
-)
-'
-<
-root
-xmlns
-=
-"
-u1
-"
->
-<
-child
-/
->
-<
-/
-root
->
-'
-)
-;
-assert_equals
-(
-serialize
-(
-parse
-(
-'
-<
-root
 xmlns
 =
 "