diff --git a/testing/web-platform/tests/custom-elements/upgrading/upgrade-custom-element-error-event.html b/testing/web-platform/tests/custom-elements/upgrading/upgrade-custom-element-error-event.html
new file mode 100644
index 0000000000000..f1c14322fa065
--- /dev/null
+++ b/testing/web-platform/tests/custom-elements/upgrading/upgrade-custom-element-error-event.html
@@ -0,0 +1,1211 @@
+<
+!
+DOCTYPE
+html
+>
+<
+link
+rel
+=
+"
+help
+"
+href
+=
+"
+https
+:
+/
+/
+dom
+.
+spec
+.
+whatwg
+.
+org
+/
+#
+concept
+-
+create
+-
+element
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+.
+.
+/
+resources
+/
+custom
+-
+elements
+-
+helpers
+.
+js
+"
+>
+<
+/
+script
+>
+<
+body
+>
+<
+script
+>
+/
+/
+Returns
+a
+promise
+which
+will
+resolve
+with
+the
+next
+error
+event
+fired
+at
+any
+/
+/
+of
+windows
+after
+the
+invocation
+of
+this
+function
+.
+Once
+one
+does
+this
+/
+/
+function
+removes
+its
+listeners
+and
+produces
+that
+error
+event
+so
+that
+it
+can
+/
+/
+be
+examined
+(
+most
+notably
+for
+which
+global
+proxy
+it
+was
+targeted
+at
+)
+.
+async
+function
+nextErrorEvent
+(
+windows
+)
+{
+let
+listener
+;
+let
+p
+=
+new
+Promise
+(
+(
+resolve
+reject
+)
+=
+>
+{
+listener
+=
+(
+event
+)
+=
+>
+{
+resolve
+(
+event
+)
+;
+event
+.
+preventDefault
+(
+)
+;
+}
+;
+}
+)
+;
+for
+(
+let
+w
+of
+windows
+)
+{
+w
+.
+addEventListener
+(
+'
+error
+'
+listener
+)
+;
+}
+try
+{
+return
+await
+p
+;
+}
+finally
+{
+for
+(
+let
+w
+of
+windows
+)
+{
+w
+.
+removeEventListener
+(
+'
+error
+'
+listener
+)
+;
+}
+}
+}
+const
+supportsCustomizedBuiltinElements
+=
+(
+function
+(
+)
+{
+let
+supported
+=
+false
+;
+customElements
+.
+define
+(
+'
+feature
+-
+detect
+'
+class
+extends
+HTMLDivElement
+{
+}
+{
+get
+extends
+(
+)
+{
+supported
+=
+true
+;
+return
+"
+div
+"
+;
+}
+}
+)
+;
+return
+supported
+;
+}
+)
+(
+)
+;
+promise_test
+(
+async
+t
+=
+>
+{
+let
+w
+=
+await
+create_window_in_test
+(
+t
+<
+script
+>
+self
+.
+MyElement
+=
+class
+extends
+HTMLElement
+{
+constructor
+(
+)
+{
+throw
+new
+Error
+(
+)
+;
+}
+}
+;
+<
+/
++
+script
+>
+)
+;
+let
+w2
+=
+await
+create_window_in_test
+(
+t
+)
+;
+w2
+.
+customElements
+.
+define
+(
+'
+my
+-
+element
+'
+w
+.
+MyElement
+)
+;
+let
+nextErrorPromise
+=
+nextErrorEvent
+(
+[
+self
+w
+w2
+]
+)
+;
+let
+elem
+=
+w2
+.
+document
+.
+createElement
+(
+'
+my
+-
+element
+'
+)
+;
+let
+errorEvent
+=
+await
+nextErrorPromise
+;
+assert_true
+(
+elem
+instanceof
+w2
+.
+HTMLUnknownElement
+'
+element
+should
+be
+an
+HTMLUnknownElement
+'
+)
+;
+if
+(
+errorEvent
+.
+error
+)
+{
+assert_true
+(
+errorEvent
+.
+error
+instanceof
+w
+.
+Error
+'
+error
+should
+be
+an
+instance
+created
+inside
+the
+MyElement
+constructor
+(
+if
+it
+was
+not
+muted
+)
+'
+)
+;
+}
+assert_equals
+(
+errorEvent
+.
+target
+w
+error
+event
+should
+target
+definition
+'
+s
+global
+but
+instead
+targets
+{
+event
+.
+target
+=
+=
+=
+w2
+?
+'
+document
+\
+'
+s
+global
+'
+:
+'
+test
+harness
+global
+'
+}
+)
+;
+}
+'
+autonomous
+:
+exception
+thrown
+in
+constructor
+is
+reported
+to
+definition
+\
+'
+s
+global
+'
+)
+;
+promise_test
+(
+async
+t
+=
+>
+{
+let
+w
+=
+await
+create_window_in_test
+(
+t
+<
+script
+>
+self
+.
+MyElement
+=
+class
+extends
+HTMLElement
+{
+constructor
+(
+)
+{
+this
+.
+textContent
+=
+'
+hello
+!
+'
+;
+}
+}
+;
+<
+/
++
+script
+>
+)
+;
+let
+w2
+=
+await
+create_window_in_test
+(
+t
+)
+;
+w2
+.
+customElements
+.
+define
+(
+'
+my
+-
+element
+'
+w
+.
+MyElement
+)
+;
+let
+nextErrorPromise
+=
+nextErrorEvent
+(
+[
+self
+w
+w2
+]
+)
+;
+let
+elem
+=
+w2
+.
+document
+.
+createElement
+(
+'
+my
+-
+element
+'
+)
+;
+let
+errorEvent
+=
+await
+nextErrorPromise
+;
+assert_true
+(
+elem
+instanceof
+w2
+.
+HTMLUnknownElement
+'
+element
+should
+be
+an
+HTMLUnknownElement
+'
+)
+;
+assert_equals
+(
+errorEvent
+.
+target
+w
+error
+event
+should
+target
+definition
+'
+s
+global
+but
+instead
+targets
+{
+event
+.
+currentTarget
+=
+=
+=
+w2
+?
+'
+document
+\
+'
+s
+global
+'
+:
+'
+test
+harness
+global
+'
+}
+)
+;
+}
+'
+autonomous
+:
+exception
+thrown
+by
+spec
+due
+to
+invalid
+element
+state
+is
+reported
+to
+definition
+\
+'
+s
+global
+'
+)
+;
+promise_test
+(
+async
+t
+=
+>
+{
+assert_implements
+(
+supportsCustomizedBuiltinElements
+'
+customized
+built
+-
+in
+elements
+not
+supported
+'
+)
+;
+let
+w
+=
+await
+create_window_in_test
+(
+t
+<
+script
+>
+self
+.
+MyDivElement
+=
+class
+extends
+HTMLDivElement
+{
+constructor
+(
+)
+{
+throw
+new
+Error
+(
+)
+;
+}
+}
+;
+<
+/
++
+script
+>
+)
+;
+let
+w2
+=
+await
+create_window_in_test
+(
+t
+)
+;
+w2
+.
+customElements
+.
+define
+(
+'
+my
+-
+div
+'
+w
+.
+MyDivElement
+{
+extends
+:
+'
+div
+'
+}
+)
+;
+let
+nextErrorPromise
+=
+nextErrorEvent
+(
+[
+self
+w
+w2
+]
+)
+;
+let
+elem
+=
+w2
+.
+document
+.
+createElement
+(
+'
+div
+'
+{
+is
+:
+'
+my
+-
+div
+'
+}
+)
+;
+let
+errorEvent
+=
+await
+nextErrorPromise
+;
+assert_equals
+(
+Object
+.
+getPrototypeOf
+(
+elem
+)
+w2
+.
+HTMLDivElement
+.
+prototype
+'
+element
+should
+be
+an
+HTMLDivElement
+(
+and
+not
+inherited
+from
+it
+)
+'
+)
+;
+assert_true
+(
+errorEvent
+.
+error
+instanceof
+w
+.
+Error
+'
+error
+should
+be
+an
+instance
+created
+inside
+the
+MyElement
+constructor
+'
+)
+;
+assert_equals
+(
+errorEvent
+.
+target
+w
+error
+event
+should
+target
+definition
+'
+s
+global
+but
+instead
+targets
+{
+event
+.
+target
+=
+=
+=
+w2
+?
+'
+document
+\
+'
+s
+global
+'
+:
+'
+test
+harness
+global
+'
+}
+)
+;
+}
+'
+customized
+built
+-
+in
+:
+exception
+thrown
+in
+constructor
+is
+reported
+to
+definition
+\
+'
+s
+global
+'
+)
+;
+promise_test
+(
+async
+t
+=
+>
+{
+assert_implements
+(
+supportsCustomizedBuiltinElements
+'
+customized
+built
+-
+in
+elements
+not
+supported
+'
+)
+;
+let
+w
+=
+await
+create_window_in_test
+(
+t
+<
+script
+>
+self
+.
+MyDivElement
+=
+class
+extends
+HTMLDivElement
+{
+constructor
+(
+)
+{
+return
+document
+.
+createElement
+(
+'
+div
+'
+)
+;
+}
+}
+;
+<
+/
++
+script
+>
+)
+;
+let
+w2
+=
+await
+create_window_in_test
+(
+t
+)
+;
+w2
+.
+customElements
+.
+define
+(
+'
+my
+-
+div
+'
+w
+.
+MyDivElement
+{
+extends
+:
+'
+div
+'
+}
+)
+;
+let
+nextErrorPromise
+=
+nextErrorEvent
+(
+[
+self
+w
+w2
+]
+)
+;
+let
+elem
+=
+w2
+.
+document
+.
+createElement
+(
+'
+div
+'
+{
+is
+:
+'
+my
+-
+div
+'
+}
+)
+;
+let
+errorEvent
+=
+await
+nextErrorPromise
+;
+assert_equals
+(
+Object
+.
+getPrototypeOf
+(
+elem
+)
+w2
+.
+HTMLDivElement
+.
+prototype
+'
+element
+should
+be
+an
+HTMLDivElement
+(
+and
+not
+inherited
+from
+it
+)
+'
+)
+;
+assert_equals
+(
+errorEvent
+.
+target
+w
+error
+event
+should
+target
+definition
+'
+s
+global
+but
+instead
+targets
+{
+event
+.
+currentTarget
+=
+=
+=
+w2
+?
+'
+document
+\
+'
+s
+global
+'
+:
+'
+test
+harness
+global
+'
+}
+)
+;
+}
+'
+customized
+built
+-
+in
+:
+exception
+thrown
+by
+spec
+due
+to
+not
+returning
+the
+same
+value
+is
+reported
+to
+definition
+\
+'
+s
+global
+'
+)
+;
+<
+/
+script
+>
+<
+/
+body
+>