diff --git a/testing/web-platform/tests/custom-elements/ElementInternals-accessibility.html b/testing/web-platform/tests/custom-elements/ElementInternals-accessibility.html
index 6a93f2d9083e..e222ef9d4664 100644
--- a/testing/web-platform/tests/custom-elements/ElementInternals-accessibility.html
+++ b/testing/web-platform/tests/custom-elements/ElementInternals-accessibility.html
@@ -174,9 +174,23 @@
 "
 ariaColIndex
 "
+/
+/
 "
 ariaColIndexText
 "
+/
+/
+tentative
+-
+>
+ElementInternals
+-
+accessibility
+.
+tentative
+.
+html
 "
 ariaColSpan
 "
@@ -189,9 +203,23 @@
 "
 ariaDescribedByElements
 "
+/
+/
 "
 ariaDescription
 "
+/
+/
+tentative
+-
+>
+ElementInternals
+-
+accessibility
+.
+tentative
+.
+html
 "
 ariaDetailsElements
 "
@@ -273,9 +301,23 @@
 "
 ariaRowIndex
 "
+/
+/
 "
 ariaRowIndexText
 "
+/
+/
+tentative
+-
+>
+ElementInternals
+-
+accessibility
+.
+tentative
+.
+html
 "
 ariaRowSpan
 "
diff --git a/testing/web-platform/tests/custom-elements/ElementInternals-accessibility.tentative.html b/testing/web-platform/tests/custom-elements/ElementInternals-accessibility.tentative.html
new file mode 100644
index 000000000000..5f9753bf15c3
--- /dev/null
+++ b/testing/web-platform/tests/custom-elements/ElementInternals-accessibility.tentative.html
@@ -0,0 +1,200 @@
+<
+!
+DOCTYPE
+HTML
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+>
+class
+TestElement
+extends
+HTMLElement
+{
+constructor
+(
+)
+{
+super
+(
+)
+;
+this
+.
+_internals
+=
+this
+.
+attachInternals
+(
+)
+;
+}
+get
+internals
+(
+)
+{
+return
+this
+.
+_internals
+;
+}
+set
+internals
+(
+val
+)
+{
+throw
+"
+Can
+'
+t
+set
+internals
+!
+"
+;
+}
+}
+customElements
+.
+define
+(
+"
+test
+-
+element
+"
+TestElement
+)
+;
+<
+/
+script
+>
+<
+test
+-
+element
+id
+=
+"
+testElement
+"
+>
+<
+/
+test
+-
+element
+>
+<
+script
+>
+const
+element
+=
+document
+.
+getElementById
+(
+"
+testElement
+"
+)
+;
+/
+/
+tentative
+properties
+const
+properties
+=
+[
+"
+ariaColIndexText
+"
+"
+ariaDescription
+"
+"
+ariaRowIndexText
+"
+]
+;
+for
+(
+const
+property
+of
+properties
+)
+{
+test
+(
+(
+)
+=
+>
+{
+assert_inherits
+(
+element
+.
+internals
+property
+)
+;
+}
+property
++
+"
+is
+defined
+in
+ElementInternals
+"
+)
+;
+}
+<
+/
+script
+>
diff --git a/testing/web-platform/tests/custom-elements/reactions/AriaMixin-string-attributes.html b/testing/web-platform/tests/custom-elements/reactions/AriaMixin-string-attributes.html
index 74330a2ce5b1..c788e3bb9c85 100644
--- a/testing/web-platform/tests/custom-elements/reactions/AriaMixin-string-attributes.html
+++ b/testing/web-platform/tests/custom-elements/reactions/AriaMixin-string-attributes.html
@@ -228,6 +228,22 @@
 <
 script
 >
+/
+/
+tentative
+-
+>
+AriaMixin
+-
+string
+-
+attributes
+.
+tentative
+.
+html
+/
+/
 testReflectAttribute
 (
 '
@@ -433,6 +449,22 @@
 '
 )
 ;
+/
+/
+tentative
+-
+>
+AriaMixin
+-
+string
+-
+attributes
+.
+tentative
+.
+html
+/
+/
 testReflectAttribute
 (
 '
@@ -502,6 +534,22 @@
 '
 )
 ;
+/
+/
+tentative
+-
+>
+AriaMixin
+-
+string
+-
+attributes
+.
+tentative
+.
+html
+/
+/
 testReflectAttribute
 (
 '
@@ -1031,6 +1079,22 @@
 '
 )
 ;
+/
+/
+tentative
+-
+>
+AriaMixin
+-
+string
+-
+attributes
+.
+tentative
+.
+html
+/
+/
 testReflectAttribute
 (
 '
diff --git a/testing/web-platform/tests/custom-elements/reactions/AriaMixin-string-attributes.tentative.html b/testing/web-platform/tests/custom-elements/reactions/AriaMixin-string-attributes.tentative.html
new file mode 100644
index 000000000000..655068b0fbf5
--- /dev/null
+++ b/testing/web-platform/tests/custom-elements/reactions/AriaMixin-string-attributes.tentative.html
@@ -0,0 +1,352 @@
+<
+!
+DOCTYPE
+html
+>
+<
+html
+>
+<
+head
+>
+<
+title
+>
+Custom
+Elements
+:
+CEReactions
+on
+Element
+interface
+<
+/
+title
+>
+<
+meta
+name
+=
+"
+author
+"
+title
+=
+"
+Ryosuke
+Niwa
+"
+href
+=
+"
+mailto
+:
+rniwa
+webkit
+.
+org
+"
+>
+<
+meta
+name
+=
+"
+assert
+"
+content
+=
+"
+String
+attributes
+of
+AriaAttributes
+interface
+must
+have
+CEReactions
+"
+>
+<
+meta
+name
+=
+"
+help
+"
+content
+=
+"
+https
+:
+/
+/
+dom
+.
+spec
+.
+whatwg
+.
+org
+/
+#
+element
+"
+>
+<
+meta
+name
+=
+"
+help
+"
+content
+=
+"
+https
+:
+/
+/
+w3c
+.
+github
+.
+io
+/
+DOM
+-
+Parsing
+/
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+.
+.
+/
+resources
+/
+custom
+-
+elements
+-
+helpers
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+.
+/
+resources
+/
+reactions
+.
+js
+"
+>
+<
+/
+script
+>
+<
+/
+head
+>
+<
+body
+>
+<
+div
+id
+=
+"
+log
+"
+>
+<
+/
+div
+>
+<
+div
+id
+=
+"
+container
+"
+>
+<
+/
+div
+>
+/
+/
+tentative
+properties
+that
+were
+added
+mid
+-
+year
+.
+.
+.
+merge
+back
+in
+after
+Interop
+2024
+completes
+<
+script
+>
+testReflectAttribute
+(
+'
+role
+'
+'
+role
+'
+'
+foo
+'
+'
+bar
+'
+'
+role
+on
+Element
+'
+)
+;
+testReflectAttribute
+(
+'
+ariaColIndexText
+'
+'
+aria
+-
+colindextext
+'
+'
+foo
+'
+'
+bar
+'
+'
+ariaColIndexText
+on
+Element
+'
+)
+;
+testReflectAttribute
+(
+'
+ariaDescription
+'
+'
+aria
+-
+description
+'
+'
+foo
+'
+'
+bar
+'
+'
+ariaDescription
+on
+Element
+'
+)
+;
+testReflectAttribute
+(
+'
+ariaRowIndexText
+'
+'
+aria
+-
+rowindextext
+'
+'
+foo
+'
+'
+bar
+'
+'
+ariaRowIndexText
+on
+Element
+'
+)
+;
+<
+/
+script
+>
+<
+/
+body
+>
+<
+/
+html
+>
diff --git a/testing/web-platform/tests/html/dom/aria-attribute-reflection.html b/testing/web-platform/tests/html/dom/aria-attribute-reflection.html
index b48fb9fe0a2e..6e43781011fb 100644
--- a/testing/web-platform/tests/html/dom/aria-attribute-reflection.html
+++ b/testing/web-platform/tests/html/dom/aria-attribute-reflection.html
@@ -1146,108 +1146,23 @@
 script
 >
 <
-div
-id
-=
-"
-colindextext
-"
-aria
+!
 -
-colindextext
-=
-"
-x
-"
->
-<
-/
-div
->
-<
-script
->
-test
-(
-function
-(
-t
-)
-{
-var
-element
-=
-document
-.
-getElementById
-(
-"
-colindextext
-"
-)
-;
-assert_equals
-(
-element
-.
-ariaColIndexText
-"
-x
-"
-)
-;
-element
-.
-ariaColIndexText
-=
-"
-y
-"
-;
-assert_equals
-(
-element
-.
-getAttribute
-(
-"
-aria
 -
 colindextext
-"
-)
-"
-y
-"
-)
-;
-testNullable
-(
-element
-"
-ariaColIndexText
-"
-"
-aria
 -
-colindextext
-"
-)
-;
-}
-"
+>
 aria
 -
-colindextext
 attribute
-reflects
+-
+reflection
 .
-"
-)
-;
-<
-/
-script
+tentative
+.
+html
+-
+-
 >
 <
 div
@@ -1458,116 +1373,23 @@
 script
 >
 <
-div
-id
-=
-"
-description
-"
-aria
+!
 -
-description
-=
-"
-cold
-as
-ice
-"
->
-<
-/
-div
->
-<
-script
->
-test
-(
-function
-(
-t
-)
-{
-var
-element
-=
-document
-.
-getElementById
-(
-"
-description
-"
-)
-;
-assert_equals
-(
-element
-.
-ariaDescription
-"
-cold
-as
-ice
-"
-)
-;
-element
-.
-ariaDescription
-=
-"
-hot
-as
-fire
-"
-;
-assert_equals
-(
-element
-.
-getAttribute
-(
-"
-aria
 -
 description
-"
-)
-"
-hot
-as
-fire
-"
-)
-;
-testNullable
-(
-element
-"
-ariaDescription
-"
-"
-aria
 -
-description
-"
-)
-;
-}
-"
+>
 aria
 -
-description
 attribute
-reflects
+-
+reflection
 .
-"
-)
-;
-<
-/
-script
+tentative
+.
+html
+-
+-
 >
 <
 div
@@ -3864,108 +3686,23 @@
 script
 >
 <
-div
-id
-=
-"
-rowindextext
-"
-aria
+!
 -
-rowindextext
-=
-"
-x
-"
->
-<
-/
-div
->
-<
-script
->
-test
-(
-function
-(
-t
-)
-{
-var
-element
-=
-document
-.
-getElementById
-(
-"
-rowindextext
-"
-)
-;
-assert_equals
-(
-element
-.
-ariaRowIndexText
-"
-x
-"
-)
-;
-element
-.
-ariaRowIndexText
-=
-"
-y
-"
-;
-assert_equals
-(
-element
-.
-getAttribute
-(
-"
-aria
 -
 rowindextext
-"
-)
-"
-y
-"
-)
-;
-testNullable
-(
-element
-"
-ariaRowIndexText
-"
-"
-aria
 -
-rowindextext
-"
-)
-;
-}
-"
+>
 aria
 -
-rowindextext
 attribute
-reflects
+-
+reflection
 .
-"
-)
-;
-<
-/
-script
+tentative
+.
+html
+-
+-
 >
 <
 div
diff --git a/testing/web-platform/tests/html/dom/aria-attribute-reflection.tentative.html b/testing/web-platform/tests/html/dom/aria-attribute-reflection.tentative.html
new file mode 100644
index 000000000000..b3ab8e1d19ab
--- /dev/null
+++ b/testing/web-platform/tests/html/dom/aria-attribute-reflection.tentative.html
@@ -0,0 +1,567 @@
+<
+!
+DOCTYPE
+HTML
+>
+<
+meta
+charset
+=
+"
+utf
+-
+8
+"
+/
+>
+<
+title
+>
+Element
+Reflection
+for
+ARIA
+properties
+<
+/
+title
+>
+<
+link
+rel
+=
+help
+href
+=
+"
+https
+:
+/
+/
+wicg
+.
+github
+.
+io
+/
+aom
+/
+spec
+/
+aria
+-
+reflection
+.
+html
+"
+>
+<
+link
+rel
+=
+"
+author
+"
+title
+=
+"
+Meredith
+Lane
+"
+href
+=
+"
+meredithl
+chromium
+.
+org
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+>
+function
+testNullable
+(
+element
+jsAttr
+contentAttr
+)
+{
+var
+originalValue
+=
+element
+[
+jsAttr
+]
+;
+assert_false
+(
+originalValue
+=
+=
+=
+null
+)
+;
+element
+[
+jsAttr
+]
+=
+null
+;
+assert_equals
+(
+element
+[
+jsAttr
+]
+null
+)
+;
+assert_false
+(
+element
+.
+hasAttribute
+(
+contentAttr
+)
+)
+;
+/
+/
+Setting
+to
+undefined
+results
+in
+same
+state
+as
+setting
+to
+null
+.
+element
+[
+jsAttr
+]
+=
+originalValue
+;
+element
+[
+jsAttr
+]
+=
+undefined
+;
+assert_equals
+(
+element
+[
+jsAttr
+]
+null
+)
+;
+assert_false
+(
+element
+.
+hasAttribute
+(
+contentAttr
+)
+)
+;
+}
+<
+/
+script
+>
+<
+!
+-
+-
+tentative
+-
+-
+>
+<
+div
+id
+=
+"
+colindextext
+"
+aria
+-
+colindextext
+=
+"
+x
+"
+>
+<
+/
+div
+>
+<
+script
+>
+test
+(
+function
+(
+t
+)
+{
+var
+element
+=
+document
+.
+getElementById
+(
+"
+colindextext
+"
+)
+;
+assert_equals
+(
+element
+.
+ariaColIndexText
+"
+x
+"
+)
+;
+element
+.
+ariaColIndexText
+=
+"
+y
+"
+;
+assert_equals
+(
+element
+.
+getAttribute
+(
+"
+aria
+-
+colindextext
+"
+)
+"
+y
+"
+)
+;
+testNullable
+(
+element
+"
+ariaColIndexText
+"
+"
+aria
+-
+colindextext
+"
+)
+;
+}
+"
+aria
+-
+colindextext
+attribute
+reflects
+.
+"
+)
+;
+<
+/
+script
+>
+<
+!
+-
+-
+tentative
+-
+-
+>
+<
+div
+id
+=
+"
+description
+"
+aria
+-
+description
+=
+"
+cold
+as
+ice
+"
+>
+<
+/
+div
+>
+<
+script
+>
+test
+(
+function
+(
+t
+)
+{
+var
+element
+=
+document
+.
+getElementById
+(
+"
+description
+"
+)
+;
+assert_equals
+(
+element
+.
+ariaDescription
+"
+cold
+as
+ice
+"
+)
+;
+element
+.
+ariaDescription
+=
+"
+hot
+as
+fire
+"
+;
+assert_equals
+(
+element
+.
+getAttribute
+(
+"
+aria
+-
+description
+"
+)
+"
+hot
+as
+fire
+"
+)
+;
+testNullable
+(
+element
+"
+ariaDescription
+"
+"
+aria
+-
+description
+"
+)
+;
+}
+"
+aria
+-
+description
+attribute
+reflects
+.
+"
+)
+;
+<
+/
+script
+>
+<
+!
+-
+-
+tentative
+-
+-
+>
+<
+div
+id
+=
+"
+rowindextext
+"
+aria
+-
+rowindextext
+=
+"
+x
+"
+>
+<
+/
+div
+>
+<
+script
+>
+test
+(
+function
+(
+t
+)
+{
+var
+element
+=
+document
+.
+getElementById
+(
+"
+rowindextext
+"
+)
+;
+assert_equals
+(
+element
+.
+ariaRowIndexText
+"
+x
+"
+)
+;
+element
+.
+ariaRowIndexText
+=
+"
+y
+"
+;
+assert_equals
+(
+element
+.
+getAttribute
+(
+"
+aria
+-
+rowindextext
+"
+)
+"
+y
+"
+)
+;
+testNullable
+(
+element
+"
+ariaRowIndexText
+"
+"
+aria
+-
+rowindextext
+"
+)
+;
+}
+"
+aria
+-
+rowindextext
+attribute
+reflects
+.
+"
+)
+;
+<
+/
+script
+>
+<
+/
+html
+>