diff --git a/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.tentative.html b/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.tentative.html
index 1606216e1cd9..31b40b055008 100644
--- a/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.tentative.html
+++ b/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.tentative.html
@@ -404,17 +404,595 @@
 cancelable
 )
 ;
+async
+function
+setup
+(
+t
+closedby
+)
+{
+t
+.
+add_cleanup
+(
+(
+)
+=
+>
+{
+dialog
+.
+close
+(
+)
+;
+dialog
+.
+removeAttribute
+(
+'
+closedby
+'
+)
+;
+dialog
+.
+returnValue
+=
+'
+'
+;
+}
+)
+;
+assert_false
+(
+dialog
+.
+hasAttribute
+(
+'
+closedby
+'
+)
+)
+;
+if
+(
+closedby
+)
+{
+dialog
+.
+setAttribute
+(
+'
+closedby
+'
+closedby
+)
+;
+}
+/
+/
+Be
+sure
+any
+pending
+close
+events
+get
+fired
+.
+await
+new
+Promise
+(
+resolve
+=
+>
+requestAnimationFrame
+(
+resolve
+)
+)
+;
+return
+getSignal
+(
+t
+)
+;
+}
+[
+false
+true
+]
+.
+forEach
+(
+modal
+=
+>
+{
+[
+null
+'
+any
+'
+'
+closedrequest
+'
+'
+none
+'
+]
+.
+forEach
+(
+closedby
+=
+>
+{
+const
+testDescription
+=
+for
+{
+modal
+?
+"
+modal
+"
+:
+"
+modeless
+"
+}
+dialog
+with
+closedby
+=
+{
+closedby
+}
+;
+promise_test
+(
+async
+(
+t
+)
+=
+>
+{
+await
+setup
+(
+t
+closedby
+)
+;
+openDialog
+(
+modal
+)
+;
+if
+(
+dialog
+.
+closedBy
+!
+=
+"
+none
+"
+)
+{
+dialog
+.
+requestClose
+(
+)
+;
+assert_false
+(
+dialog
+.
+open
+)
+;
+}
+else
+{
+assert_throws_dom
+(
+'
+InvalidStateError
+'
+(
+)
+=
+>
+dialog
+.
+requestClose
+(
+)
+)
+;
+assert_true
+(
+dialog
+.
+open
+)
+;
+}
+}
+requestClose
+basic
+behavior
+{
+testDescription
+}
+)
+;
+promise_test
+(
+async
+(
+t
+)
+=
+>
+{
+const
+signal
+=
+await
+setup
+(
+t
+closedby
+)
+;
+let
+events
+=
+[
+]
+;
+dialog
+.
+addEventListener
+(
+'
+cancel
+'
+(
+)
+=
+>
+events
+.
+push
+(
+'
+cancel
+'
+)
+{
+signal
+}
+)
+;
+dialog
+.
+addEventListener
+(
+'
+close
+'
+(
+)
+=
+>
+events
+.
+push
+(
+'
+close
+'
+)
+{
+signal
+}
+)
+;
+openDialog
+(
+modal
+)
+;
+assert_array_equals
+(
+events
+[
+]
+)
+;
+if
+(
+dialog
+.
+closedBy
+!
+=
+"
+none
+"
+)
+{
+dialog
+.
+requestClose
+(
+)
+;
+assert_false
+(
+dialog
+.
+open
+)
+;
+assert_array_equals
+(
+events
+[
+'
+cancel
+'
+]
+'
+close
+is
+scheduled
+'
+)
+;
+await
+new
+Promise
+(
+resolve
+=
+>
+requestAnimationFrame
+(
+resolve
+)
+)
+;
+assert_array_equals
+(
+events
+[
+'
+cancel
+'
+'
+close
+'
+]
+)
+;
+}
+else
+{
+assert_throws_dom
+(
+'
+InvalidStateError
+'
+(
+)
+=
+>
+dialog
+.
+requestClose
+(
+)
+)
+;
+}
+}
+requestClose
+fires
+both
+cancel
+and
+close
+{
+testDescription
+}
+)
+;
+promise_test
+(
+async
+(
+t
+)
+=
+>
+{
+const
+signal
+=
+await
+setup
+(
+t
+'
+none
+'
+)
+;
+let
+events
+=
 [
-false
-true
 ]
+;
+dialog
 .
-forEach
+addEventListener
+(
+'
+cancel
+'
+(
+)
+=
+>
+events
+.
+push
+(
+'
+cancel
+'
+)
+{
+signal
+}
+)
+;
+dialog
+.
+addEventListener
+(
+'
+close
+'
+(
+)
+=
+>
+events
+.
+push
+(
+'
+close
+'
+)
+{
+signal
+}
+)
+;
+openDialog
 (
 modal
+)
+;
+dialog
+.
+setAttribute
+(
+'
+closedby
+'
+closedby
+)
+;
+assert_array_equals
+(
+events
+[
+]
+)
+;
+if
+(
+dialog
+.
+closedBy
+!
+=
+"
+none
+"
+)
+{
+dialog
+.
+requestClose
+(
+)
+;
+assert_false
+(
+dialog
+.
+open
+)
+;
+}
+else
+{
+assert_throws_dom
+(
+'
+InvalidStateError
+'
+(
+)
 =
 >
+dialog
+.
+requestClose
+(
+)
+)
+;
+}
+}
+changing
+closedby
+from
+'
+none
+'
+to
+'
+{
+closedby
+}
+'
+for
 {
+modal
+?
+"
+modal
+"
+:
+"
+modeless
+"
+}
+dialog
+)
+;
 promise_test
 (
 async
@@ -424,19 +1002,68 @@
 =
 >
 {
+const
+signal
+=
+await
+setup
+(
 t
+closedby
+)
+;
+let
+events
+=
+[
+]
+;
+dialog
 .
-add_cleanup
+addEventListener
 (
+'
+cancel
+'
 (
 )
 =
 >
+events
+.
+push
+(
+'
+cancel
+'
+)
+{
+signal
+}
+)
+;
 dialog
 .
+addEventListener
+(
+'
 close
+'
+(
+)
+=
+>
+events
+.
+push
 (
+'
+close
+'
 )
+{
+signal
+}
 )
 ;
 openDialog
@@ -446,6 +1073,34 @@
 ;
 dialog
 .
+removeAttribute
+(
+'
+closedby
+'
+)
+;
+assert_array_equals
+(
+events
+[
+]
+)
+;
+if
+(
+dialog
+.
+closedBy
+!
+=
+"
+none
+"
+)
+{
+dialog
+.
 requestClose
 (
 )
@@ -458,27 +1113,63 @@
 )
 ;
 }
+else
+{
+assert_throws_dom
+(
+'
+InvalidStateError
+'
+(
+)
+=
+>
+dialog
+.
+requestClose
+(
+)
+)
+;
+}
+}
+Removing
+closedby
+when
+closedby
+=
+'
+{
+closedby
+}
+'
+for
 {
 modal
 ?
 "
-Modal
-:
+modal
 "
 :
 "
-Non
--
-modal
-:
+modeless
 "
 }
-requestClose
-closes
-the
 dialog
 )
-;
+;
+if
+(
+dialog
+.
+closedBy
+!
+=
+"
+none
+"
+)
+{
 promise_test
 (
 async
@@ -488,27 +1179,14 @@
 =
 >
 {
-t
-.
-add_cleanup
-(
-(
-)
-=
->
-dialog
-.
-close
-(
-)
-)
-;
 const
 signal
 =
-getSignal
+await
+setup
 (
 t
+closedby
 )
 ;
 let
@@ -628,25 +1306,295 @@
 )
 ;
 }
+requestClose
+can
+be
+cancelled
 {
+testDescription
+}
+)
+;
+promise_test
+(
+async
+(
+t
+)
+=
+>
+{
+await
+setup
+(
+t
+closedby
+)
+;
+openDialog
+(
 modal
-?
-"
-Modal
-:
-"
-:
-"
-Non
--
-modal
+)
+;
+assert_equals
+(
+dialog
+.
+returnValue
+'
+'
+'
+Return
+value
+starts
+out
+empty
+'
+)
+;
+const
+returnValue
+=
+'
+The
+return
+value
+'
+;
+dialog
+.
+requestClose
+(
+returnValue
+)
+;
+assert_false
+(
+dialog
+.
+open
+)
+;
+assert_equals
+(
+dialog
+.
+returnValue
+returnValue
+'
+Return
+value
+should
+be
+set
+'
+)
+;
+dialog
+.
+show
+(
+)
+;
+dialog
+.
+close
+(
+)
+;
+assert_equals
+(
+dialog
+.
+returnValue
+returnValue
+'
+Return
+value
+should
+not
+be
+changed
+by
+close
+(
+)
+'
+)
+;
+dialog
+.
+show
+(
+)
+;
+dialog
+.
+close
+(
+'
+another
+'
+)
+;
+assert_equals
+(
+dialog
+.
+returnValue
+'
+another
+'
+'
+Return
+value
+changes
+via
+close
+(
+value
+)
+'
+)
+;
+}
+requestClose
+(
+returnValue
+)
+passes
+along
+the
+return
+value
+{
+testDescription
+}
+)
+;
+promise_test
+(
+async
+(
+t
+)
+=
+>
+{
+await
+setup
+(
+t
+closedby
+)
+;
+dialog
+.
+addEventListener
+(
+'
+cancel
+'
+(
+e
+)
+=
+>
+e
+.
+preventDefault
+(
+)
+{
+once
 :
-"
+true
 }
+)
+;
+openDialog
+(
+modal
+)
+;
+dialog
+.
+returnValue
+=
+'
+foo
+'
+;
+assert_equals
+(
+dialog
+.
+returnValue
+'
+foo
+'
+)
+;
+dialog
+.
 requestClose
-can
+(
+'
+This
+should
+not
+get
+saved
+'
+)
+;
+assert_true
+(
+dialog
+.
+open
+'
+cancelled
+'
+)
+;
+assert_equals
+(
+dialog
+.
+returnValue
+'
+foo
+'
+'
+Return
+value
+should
+not
 be
+changed
+'
+)
+;
+}
+requestClose
+(
+returnValue
+)
+doesn
+'
+t
+change
+returnvalue
+when
 cancelled
+{
+testDescription
+}
+)
+;
+}
+}
 )
 ;
 }