diff --git a/testing/web-platform/meta/MANIFEST.json b/testing/web-platform/meta/MANIFEST.json
index f3afe6904840c..6d30da333b0aa 100644
--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -3750757,7 +3750757,7 @@ md
 :
 [
 "
-45475b5c261fb5a3fe571fd1c4151e7e3507cf67
+67801e31fa6bf99a22a5407a7e59567e714ee7c3
 "
 "
 support
diff --git a/testing/web-platform/tests/docs/_writing-tests/testharness.md b/testing/web-platform/tests/docs/_writing-tests/testharness.md
index c6e79b2395c82..f974eeb958ca5 100644
--- a/testing/web-platform/tests/docs/_writing-tests/testharness.md
+++ b/testing/web-platform/tests/docs/_writing-tests/testharness.md
@@ -207,11 +207,17 @@ the
 URL
 .
 There
-is
-a
+are
+two
 utility
-script
+scripts
 in
+that
+work
+well
+together
+with
+variants
 /
 common
 /
@@ -220,12 +226,19 @@ subset
 tests
 .
 js
-that
-works
-well
-together
-with
-variants
+and
+/
+common
+/
+subset
+-
+tests
+-
+by
+-
+key
+.
+js
 where
 a
 test
@@ -415,6 +428,180 @@ name
 /
 script
 >
+With
+subsetTestByKey
+the
+key
+is
+given
+as
+the
+first
+argument
+and
+the
+query
+string
+can
+include
+or
+exclude
+a
+key
+(
+will
+be
+matched
+as
+a
+regular
+expression
+)
+.
+<
+!
+doctype
+html
+>
+<
+title
+>
+Testing
+variants
+by
+key
+<
+/
+title
+>
+<
+meta
+name
+=
+"
+variant
+"
+content
+=
+"
+?
+include
+=
+Foo
+"
+>
+<
+meta
+name
+=
+"
+variant
+"
+content
+=
+"
+?
+include
+=
+Bar
+"
+>
+<
+meta
+name
+=
+"
+variant
+"
+content
+=
+"
+?
+exclude
+=
+(
+Foo
+|
+Bar
+)
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+script
+src
+=
+"
+/
+common
+/
+subset
+-
+tests
+-
+by
+-
+key
+.
+js
+"
+>
+<
+script
+>
+subsetTestByKey
+(
+"
+Foo
+"
+async_test
+(
+)
+=
+>
+{
+.
+.
+.
+}
+"
+Testing
+foo
+"
+)
+;
+.
+.
+.
+<
+/
+script
+>
 #
 #
 Auto