diff --git a/moli-renderer-v8/src/context_bootstrap/event_legacy.rs b/moli-renderer-v8/src/context_bootstrap/event_legacy.rs
index 5ed7e90392..20d5745057 100644
--- a/moli-renderer-v8/src/context_bootstrap/event_legacy.rs
+++ b/moli-renderer-v8/src/context_bootstrap/event_legacy.rs
@@ -124,33 +124,6 @@ struct InitMouseEventArgs<'s> {
     related_target: Option<v8::Local<'s, v8::Object>>,
 }
 
-#[derive(webidl::WebIdlArgs)]
-#[webidl(prefix = "KeyboardEvent.initKeyboardEvent")]
-struct InitKeyboardEventArgs<'s> {
-    #[webidl(
-        required,
-        converter = "raw",
-        missing_message = "Failed to execute 'initKeyboardEvent': 1 argument required."
-    )]
-    event_type: webidl::DomString16,
-    #[webidl(default = false)]
-    bubbles: bool,
-    #[webidl(default = false)]
-    cancelable: bool,
-    #[webidl(index = 3, converter = "raw")]
-    view: Option<v8::Local<'s, v8::Value>>,
-    #[webidl(default = "", index = 4)]
-    key: String,
-    #[webidl(default = 0, index = 5)]
-    location: i32,
-    #[webidl(default = "", index = 6)]
-    _modifiers_list: String,
-    #[webidl(default = false, index = 7)]
-    repeat: bool,
-    #[webidl(default = "", index = 8)]
-    _locale: String,
-}
-
 #[derive(webidl::WebIdlArgs)]
 #[webidl(prefix = "TextEvent.initTextEvent")]
 struct InitTextEventArgs<'s> {
@@ -189,13 +162,6 @@ struct InitCompositionEventArgs<'s> {
     data: String,
 }
 
-#[derive(webidl::WebIdlArgs)]
-#[webidl(prefix = "KeyboardEvent.getModifierState")]
-struct KeyboardEventGetModifierStateArgs {
-    #[webidl(required)]
-    key_arg: String,
-}
-
 #[derive(WebApiObject)]
 #[webapi(plain, data_properties, enumerable)]
 struct LegacyUiEventInitDeclaration<'scope> {
@@ -243,28 +209,6 @@ struct LegacyMouseEventTailInitDeclaration<'scope> {
     related_target: v8::Local<'scope, v8::Value>,
 }
 
-#[derive(WebApiObject)]
-#[webapi(plain, data_properties, enumerable)]
-struct LegacyKeyboardEventInitDeclaration<'scope> {
-    view: v8::Local<'scope, v8::Value>,
-    #[webapi(constructor_default = 0)]
-    detail: i32,
-    key: v8::Local<'scope, v8::String>,
-    code: v8::Local<'scope, v8::String>,
-    location: i32,
-    repeat: bool,
-    #[webapi(constructor_default = false)]
-    is_composing: bool,
-    #[webapi(constructor_default = false)]
-    ctrl_key: bool,
-    #[webapi(constructor_default = false)]
-    shift_key: bool,
-    #[webapi(constructor_default = false)]
-    alt_key: bool,
-    #[webapi(constructor_default = false)]
-    meta_key: bool,
-}
-
 #[derive(WebApiObject)]
 #[webapi(plain, data_properties, enumerable)]
 struct LegacyCustomEventInitDeclaration<'scope> {
@@ -420,32 +364,6 @@ pub(super) fn mouse_event_init_callback<'s>(
     .expect("legacy MouseEvent tail init declaration should initialize");
 }
 
-pub(super) fn keyboard_event_init_callback<'s>(
-    scope: &mut v8::PinScope<'s, '_>,
-    args: v8::FunctionCallbackArguments<'s>,
-    _rv: v8::ReturnValue<'_, v8::Value>,
-) {
-    let event = super::events::event_backing(scope, args.this());
-    let Some(parsed) = webidl::parse_args::<InitKeyboardEventArgs>(scope, &args) else {
-        return;
-    };
-    let view = legacy_event_view_or_global(scope, parsed.view);
-    if !reinitialize_event_object(
-        scope,
-        event,
-        &parsed.event_type.0,
-        parsed.bubbles,
-        parsed.cancelable,
-    ) {
-        return;
-    }
-    let key = v8_string(scope, &parsed.key).unwrap_or_else(|| v8str(scope, ""));
-    let code = v8str(scope, "");
-    LegacyKeyboardEventInitDeclaration::new(view, key, code, parsed.location, parsed.repeat)
-        .initialize(scope, event)
-        .expect("legacy KeyboardEvent init declaration should initialize");
-}
-
 pub(super) fn composition_event_init_callback<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     args: v8::FunctionCallbackArguments<'s>,
@@ -523,38 +441,3 @@ pub(super) fn storage_event_init_callback<'s>(
         parsed.storage_area.map(|value| value.0.into()),
     );
 }
-
-pub(super) fn keyboard_event_get_modifier_state_callback<'s>(
-    scope: &mut v8::PinScope<'s, '_>,
-    args: v8::FunctionCallbackArguments<'s>,
-    mut rv: v8::ReturnValue<'_, v8::Value>,
-) {
-    let Some(parsed) = webidl::parse_args::<KeyboardEventGetModifierStateArgs>(scope, &args) else {
-        return;
-    };
-    let event = super::events::event_backing(scope, args.this());
-    let key_name = match parsed.key_arg.as_str() {
-        "Alt" | "AltGraph" => "altKey",
-        "Control" => "ctrlKey",
-        "Shift" => "shiftKey",
-        "Meta" => "metaKey",
-        "Accel" => {
-            let ctrl = event
-                .get(scope, v8str(scope, "ctrlKey").into())
-                .is_some_and(|value| value.boolean_value(scope));
-            let meta = event
-                .get(scope, v8str(scope, "metaKey").into())
-                .is_some_and(|value| value.boolean_value(scope));
-            rv.set(v8::Boolean::new(scope, ctrl || meta).into());
-            return;
-        }
-        _ => {
-            rv.set(v8::Boolean::new(scope, false).into());
-            return;
-        }
-    };
-    let value = event
-        .get(scope, v8str(scope, key_name).into())
-        .is_some_and(|value| value.boolean_value(scope));
-    rv.set(v8::Boolean::new(scope, value).into());
-}
diff --git a/moli-renderer-v8/src/context_bootstrap/event_template.rs b/moli-renderer-v8/src/context_bootstrap/event_template.rs
index 653379171e..13d8d5e1e9 100644
--- a/moli-renderer-v8/src/context_bootstrap/event_template.rs
+++ b/moli-renderer-v8/src/context_bootstrap/event_template.rs
@@ -2,7 +2,6 @@ use super::{
     event_document::{document_create_event_callback, document_has_focus_callback},
     event_legacy::{
         composition_event_init_callback, custom_event_init_callback, event_init_event_callback,
-        keyboard_event_get_modifier_state_callback, keyboard_event_init_callback,
         mouse_event_init_callback, storage_event_init_callback, text_event_init_callback,
         ui_event_init_callback,
     },
@@ -13,16 +12,18 @@ use super::{
         event_cancelable_getter_function, event_composed_getter_function,
         event_composed_path_callback, event_current_target_getter_function,
         event_default_prevented_getter_function, event_event_phase_getter_function,
+        event_get_modifier_state_callback, event_platform_attribute_getter,
         event_prevent_default_callback, event_return_value_getter_function,
         event_return_value_setter_function, event_src_element_getter_function,
         event_stop_immediate_propagation_callback, event_stop_propagation_callback,
         event_target_getter_function, event_time_stamp_getter_function, event_type_getter_function,
         event_value_attribute_getter, focus_event_related_target_getter_function,
-        form_data_event_form_data_getter_function, message_event_init_callback,
-        mouse_event_related_target_getter_function, pointer_event_get_predicted_events_callback,
-        submit_event_agent_invoked_getter_function, submit_event_respond_with_callback,
-        submit_event_submitter_getter_function, track_event_track_getter_function,
-        ui_event_pseudo_target_getter_function, ui_event_which_getter_function,
+        form_data_event_form_data_getter_function, keyboard_event_init_callback,
+        message_event_init_callback, mouse_event_related_target_getter_function,
+        pointer_event_get_predicted_events_callback, submit_event_agent_invoked_getter_function,
+        submit_event_respond_with_callback, submit_event_submitter_getter_function,
+        track_event_track_getter_function, ui_event_pseudo_target_getter_function,
+        ui_event_which_getter_function,
     },
     selection_surface::document_get_selection_callback,
     specs::ConstructorSpec,
@@ -166,7 +167,7 @@ struct FormDataEventTemplateAccessorsDeclaration {
 #[derive(WebApiFunctionTemplate)]
 #[webapi(interface = web_api_interfaces::UIEvent, enumerable, receiver)]
 struct UiEventTemplateMethodsDeclaration {
-    #[webapi(accessor_property = "view", getter = event_value_attribute_getter, data = crate::util::v8str(scope, "view"))]
+    #[webapi(accessor_property = "view", getter = event_platform_attribute_getter, data = crate::util::v8str(scope, "view"))]
     view: (),
     #[webapi(accessor_property = "detail", getter = event_value_attribute_getter, data = crate::util::v8str(scope, "detail"))]
     detail: (),
@@ -301,8 +302,8 @@ struct KeyboardEventTemplateMethodsDeclaration {
 
     #[webapi(
         method = "getModifierState",
-        length = 0,
-        callback = keyboard_event_get_modifier_state_callback
+        length = 1,
+        callback = event_get_modifier_state_callback
     )]
     get_modifier_state: (),
 }
diff --git a/moli-renderer-v8/src/context_bootstrap/events.rs b/moli-renderer-v8/src/context_bootstrap/events.rs
index d0df2f19b7..d09f6f0ee9 100644
--- a/moli-renderer-v8/src/context_bootstrap/events.rs
+++ b/moli-renderer-v8/src/context_bootstrap/events.rs
@@ -9,8 +9,10 @@ mod kind;
 mod message;
 pub(crate) use message::construct_original_message_event;
 mod methods;
+mod modifiers;
 mod subclasses;
 mod submit;
+mod ui;
 mod value;
 mod wrappers;
 
@@ -499,7 +501,12 @@ pub(crate) use wrappers::{
 };
 
 pub(in crate::context_bootstrap) use message::message_event_init_callback;
-pub(in crate::context_bootstrap) use wrappers::event_value_attribute_getter;
+pub(in crate::context_bootstrap) use wrappers::{
+    event_platform_attribute_getter, event_value_attribute_getter,
+};
+
+pub(in crate::context_bootstrap) use modifiers::event_get_modifier_state_callback;
+pub(in crate::context_bootstrap) use subclasses::keyboard_event_init_callback;
 
 pub(crate) use subclasses::construct_original_error_event;
 
diff --git a/moli-renderer-v8/src/context_bootstrap/events/modifiers.rs b/moli-renderer-v8/src/context_bootstrap/events/modifiers.rs
new file mode 100644
index 0000000000..ec5505d6cd
--- /dev/null
+++ b/moli-renderer-v8/src/context_bootstrap/events/modifiers.rs
@@ -0,0 +1,153 @@
+use super::*;
+use crate::webidl;
+
+const EVENT_MODIFIERS_SLOT: &str = "__moliEventModifiers";
+const MODIFIER_KEYS: [&str; 14] = [
+    "Alt",
+    "Control",
+    "Meta",
+    "AltGraph",
+    "CapsLock",
+    "Fn",
+    "FnLock",
+    "Hyper",
+    "NumLock",
+    "ScrollLock",
+    "Super",
+    "Symbol",
+    "SymbolLock",
+    "Shift",
+];
+
+/// EventModifierInit members, in Web IDL dictionary order.
+#[derive(Default, webidl::WebIdlDictionary)]
+#[webidl(prefix = "EventModifierInit")]
+pub(super) struct EventModifierInitMembers {
+    #[webidl(default = false)]
+    alt_key: bool,
+    #[webidl(default = false)]
+    ctrl_key: bool,
+    #[webidl(default = false)]
+    meta_key: bool,
+    #[webidl(default = false)]
+    modifier_alt_graph: bool,
+    #[webidl(default = false)]
+    modifier_caps_lock: bool,
+    #[webidl(default = false)]
+    modifier_fn: bool,
+    #[webidl(default = false)]
+    modifier_fn_lock: bool,
+    #[webidl(default = false)]
+    modifier_hyper: bool,
+    #[webidl(default = false)]
+    modifier_num_lock: bool,
+    #[webidl(default = false)]
+    modifier_scroll_lock: bool,
+    #[webidl(default = false)]
+    modifier_super: bool,
+    #[webidl(default = false)]
+    modifier_symbol: bool,
+    #[webidl(default = false)]
+    modifier_symbol_lock: bool,
+    #[webidl(default = false)]
+    shift_key: bool,
+}
+
+#[derive(WebApiObject)]
+#[webapi(plain, data_properties, enumerable)]
+struct EventModifierProperties {
+    alt_key: bool,
+    ctrl_key: bool,
+    meta_key: bool,
+    shift_key: bool,
+}
+
+impl EventModifierInitMembers {
+    pub(super) fn initialize<'s>(
+        &self,
+        scope: &mut v8::PinScope<'s, '_>,
+        event: v8::Local<'s, v8::Object>,
+    ) {
+        let event = event_backing(scope, event);
+        let values = [
+            self.alt_key,
+            self.ctrl_key,
+            self.meta_key,
+            self.modifier_alt_graph,
+            self.modifier_caps_lock,
+            self.modifier_fn,
+            self.modifier_fn_lock,
+            self.modifier_hyper,
+            self.modifier_num_lock,
+            self.modifier_scroll_lock,
+            self.modifier_super,
+            self.modifier_symbol,
+            self.modifier_symbol_lock,
+            self.shift_key,
+        ];
+        let bits = values
+            .into_iter()
+            .enumerate()
+            .fold(0, |bits, (index, active)| {
+                bits | (u32::from(active) << index)
+            });
+        set_private_value(
+            scope,
+            event,
+            EVENT_MODIFIERS_SLOT,
+            v8::Integer::new_from_unsigned(scope, bits).into(),
+        );
+        EventModifierProperties::new(self.alt_key, self.ctrl_key, self.meta_key, self.shift_key)
+            .initialize(scope, event)
+            .expect("event modifier properties should initialize");
+    }
+}
+
+pub(in crate::context_bootstrap) fn initialize_legacy_event_modifiers<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    event: v8::Local<'s, v8::Object>,
+    ctrl_key: bool,
+    alt_key: bool,
+    shift_key: bool,
+    meta_key: bool,
+) {
+    EventModifierInitMembers {
+        ctrl_key,
+        alt_key,
+        shift_key,
+        meta_key,
+        ..Default::default()
+    }
+    .initialize(scope, event);
+}
+
+#[derive(webidl::WebIdlArgs)]
+#[webidl(prefix = "getModifierState")]
+struct GetModifierStateArgs {
+    #[webidl(required)]
+    key_arg: String,
+}
+
+pub(in crate::context_bootstrap) fn event_get_modifier_state_callback<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    args: v8::FunctionCallbackArguments<'s>,
+    mut rv: v8::ReturnValue<'_, v8::Value>,
+) {
+    let Some(parsed) = webidl::parse_args::<GetModifierStateArgs>(scope, &args) else {
+        return;
+    };
+    // Retain the legacy accelerator alias for the native platform.
+    let key = match parsed.key_arg.as_str() {
+        "Accel" if cfg!(target_os = "macos") => "Meta",
+        "Accel" => "Control",
+        key => key,
+    };
+    let bits = event_private_value(scope, args.this(), EVENT_MODIFIERS_SLOT)
+        .and_then(|value| value.uint32_value(scope))
+        .unwrap_or_default();
+    let active = MODIFIER_KEYS
+        .iter()
+        .position(|name| *name == key)
+        .is_some_and(|index| bits & (1 << index) != 0);
+    rv.set(v8::Boolean::new(scope, active).into());
+}
diff --git a/moli-renderer-v8/src/context_bootstrap/events/subclasses.rs b/moli-renderer-v8/src/context_bootstrap/events/subclasses.rs
index 1838a9f11e..456d87202c 100644
--- a/moli-renderer-v8/src/context_bootstrap/events/subclasses.rs
+++ b/moli-renderer-v8/src/context_bootstrap/events/subclasses.rs
@@ -26,3 +26,4 @@ pub(in crate::context_bootstrap) use pointer::{
 pub(in crate::context_bootstrap::events) use data::initialize_navigate_event_methods;
 
 pub(crate) use error::construct_original_error_event;
+pub(in crate::context_bootstrap) use keyboard::keyboard_event_init_callback;
diff --git a/moli-renderer-v8/src/context_bootstrap/events/subclasses/constructor.rs b/moli-renderer-v8/src/context_bootstrap/events/subclasses/constructor.rs
index 62701aeaf1..4a173f2731 100644
--- a/moli-renderer-v8/src/context_bootstrap/events/subclasses/constructor.rs
+++ b/moli-renderer-v8/src/context_bootstrap/events/subclasses/constructor.rs
@@ -51,6 +51,14 @@ fn event_subclass_constructor_callback<'s>(
     } else {
         None
     };
+    let keyboard_event_init = if kind == EventSubclassKind::KeyboardEvent {
+        let Some(init) = keyboard::parse_keyboard_event_init(scope, &args) else {
+            return;
+        };
+        Some(init)
+    } else {
+        None
+    };
     let security_policy_init = if kind == EventSubclassKind::SecurityPolicyViolationEvent {
         let Some(init) = security_policy::parse_security_policy_violation_event_init(scope, &args)
         else {
@@ -80,6 +88,11 @@ fn event_subclass_constructor_callback<'s>(
         .as_ref()
         .map(security_policy::SecurityPolicyViolationEventInit::event_flags)
         .or_else(|| error_init.as_ref().map(error::ErrorEventInit::event_flags))
+        .or_else(|| {
+            keyboard_event_init
+                .as_ref()
+                .map(keyboard::KeyboardEventInit::event_flags)
+        })
         .or_else(|| {
             storage_event_init
                 .as_ref()
@@ -141,9 +154,11 @@ fn event_subclass_constructor_callback<'s>(
             }
         }
         EventSubclassKind::KeyboardEvent => {
-            if !keyboard::initialize_keyboard_event(scope, event, init) {
-                return;
-            }
+            keyboard::initialize_keyboard_event(
+                scope,
+                event,
+                keyboard_event_init.expect("KeyboardEvent init should be parsed"),
+            );
         }
         EventSubclassKind::InputEvent => data::initialize_input_event(scope, event, init),
         EventSubclassKind::WheelEvent => {
diff --git a/moli-renderer-v8/src/context_bootstrap/events/subclasses/keyboard.rs b/moli-renderer-v8/src/context_bootstrap/events/subclasses/keyboard.rs
index 50ea026123..f143d0a618 100644
--- a/moli-renderer-v8/src/context_bootstrap/events/subclasses/keyboard.rs
+++ b/moli-renderer-v8/src/context_bootstrap/events/subclasses/keyboard.rs
@@ -1,56 +1,179 @@
+use super::super::ui::{UiEventInit, initialize_legacy_ui_event};
 use super::*;
+use crate::webidl;
 use moli_webapi_declare::WebApiObject;
 
+/// Own KeyboardEventInit members, including the legacy code members.
+#[derive(webidl::WebIdlDictionary)]
+#[webidl(prefix = "KeyboardEventInit")]
+struct KeyboardEventInitMembers {
+    #[webidl(default = 0)]
+    char_code: u32,
+    #[webidl(default = webidl::DomString16(Vec::new()), converter = "raw")]
+    code: webidl::DomString16,
+    #[webidl(default = false)]
+    is_composing: bool,
+    #[webidl(default = webidl::DomString16(Vec::new()), converter = "raw")]
+    key: webidl::DomString16,
+    #[webidl(default = 0)]
+    key_code: u32,
+    #[webidl(default = 0)]
+    location: u32,
+    #[webidl(default = false)]
+    repeat: bool,
+}
+
+impl Default for KeyboardEventInitMembers {
+    fn default() -> Self {
+        Self {
+            char_code: 0,
+            code: webidl::DomString16(Vec::new()),
+            is_composing: false,
+            key: webidl::DomString16(Vec::new()),
+            key_code: 0,
+            location: 0,
+            repeat: false,
+        }
+    }
+}
+
+#[derive(Default)]
+pub(super) struct KeyboardEventInit<'s> {
+    ui: UiEventInit<'s>,
+    modifiers: super::super::modifiers::EventModifierInitMembers,
+    keyboard: KeyboardEventInitMembers,
+}
+
+impl KeyboardEventInit<'_> {
+    pub(super) fn event_flags(&self) -> (bool, bool, bool) {
+        self.ui.event_flags()
+    }
+}
+
+pub(super) fn parse_keyboard_event_init<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    args: &v8::FunctionCallbackArguments<'s>,
+) -> Option<KeyboardEventInit<'s>> {
+    let parsed = webidl::dictionary_arg(args, 1, webidl::Context::argument("KeyboardEvent", 2))
+        .and_then(|object| {
+            let Some(object) = object else {
+                return Ok(KeyboardEventInit::default());
+            };
+            // Parse the inheritance chain base-first. Each group's members are
+            // read once in lexical order, before initializing any event state.
+            Ok(KeyboardEventInit {
+                ui: webidl::parse_dictionary_object(scope, object)?,
+                modifiers: webidl::parse_dictionary_object(scope, object)?,
+                keyboard: webidl::parse_dictionary_object(scope, object)?,
+            })
+        });
+    match parsed {
+        Ok(init) => Some(init),
+        Err(error) => {
+            webidl::throw_error(scope, &error);
+            None
+        }
+    }
+}
+
 #[derive(WebApiObject)]
 #[webapi(plain, data_properties, enumerable)]
 struct KeyboardEventInitDeclaration<'scope> {
-    view: v8::Local<'scope, v8::Value>,
-    detail: f64,
     key: v8::Local<'scope, v8::String>,
     code: v8::Local<'scope, v8::String>,
-    location: f64,
-    char_code: f64,
-    key_code: f64,
-    which: f64,
+    location: u32,
+    char_code: u32,
+    key_code: u32,
     repeat: bool,
     is_composing: bool,
-    ctrl_key: bool,
-    shift_key: bool,
-    alt_key: bool,
-    meta_key: bool,
 }
 
-pub(in crate::context_bootstrap::events::subclasses) fn initialize_keyboard_event<'s>(
+pub(super) fn initialize_keyboard_event<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     event: v8::Local<'s, v8::Object>,
-    init: Option<v8::Local<'s, v8::Object>>,
-) -> bool {
-    let Ok(view) = init_window_view_property(scope, init, "KeyboardEvent") else {
-        return false;
-    };
-    let detail = init_number_property(scope, init, "detail", 0.0);
-    let key = init_string_property(scope, init, "key", "");
-    let code = init_string_property(scope, init, "code", "");
-    let key = v8_string(scope, &key).expect("KeyboardEvent key");
-    let code = v8_string(scope, &code).expect("KeyboardEvent code");
-    let location = init_number_property(scope, init, "location", 0.0);
+    parsed: KeyboardEventInit<'s>,
+) {
+    parsed.ui.initialize(scope, event);
+    let key = v8_string_from_utf16_units(scope, &parsed.keyboard.key.0).expect("KeyboardEvent key");
+    let code =
+        v8_string_from_utf16_units(scope, &parsed.keyboard.code.0).expect("KeyboardEvent code");
+    parsed.modifiers.initialize(scope, event);
     KeyboardEventInitDeclaration::new(
-        view,
-        detail,
         key,
         code,
-        location,
-        init_number_property(scope, init, "charCode", 0.0),
-        init_number_property(scope, init, "keyCode", 0.0),
-        init_number_property(scope, init, "which", 0.0),
-        init_bool_property(scope, init, "repeat", false),
-        init_bool_property(scope, init, "isComposing", false),
-        init_bool_property(scope, init, "ctrlKey", false),
-        init_bool_property(scope, init, "shiftKey", false),
-        init_bool_property(scope, init, "altKey", false),
-        init_bool_property(scope, init, "metaKey", false),
+        parsed.keyboard.location,
+        parsed.keyboard.char_code,
+        parsed.keyboard.key_code,
+        parsed.keyboard.repeat,
+        parsed.keyboard.is_composing,
     )
     .initialize(scope, event)
     .expect("KeyboardEvent init declaration should initialize");
-    true
+}
+
+#[derive(webidl::WebIdlArgs)]
+#[webidl(prefix = "KeyboardEvent.initKeyboardEvent")]
+struct InitKeyboardEventArgs<'s> {
+    #[webidl(required, converter = "raw")]
+    event_type: webidl::DomString16,
+    #[webidl(default = false)]
+    bubbles: bool,
+    #[webidl(default = false)]
+    cancelable: bool,
+    #[webidl(nullable, interface = crate::web_api_interfaces::Window, brand_check = crate::context_bootstrap::is_window_receiver)]
+    view: Option<v8::Local<'s, v8::Object>>,
+    #[webidl(default = webidl::DomString16(Vec::new()), converter = "raw")]
+    key: webidl::DomString16,
+    #[webidl(default = 0)]
+    location: u32,
+    #[webidl(default = false)]
+    ctrl_key: bool,
+    #[webidl(default = false)]
+    alt_key: bool,
+    #[webidl(default = false)]
+    shift_key: bool,
+    #[webidl(default = false)]
+    meta_key: bool,
+}
+
+#[derive(WebApiObject)]
+#[webapi(plain, data_properties, enumerable)]
+struct LegacyKeyboardEventInitDeclaration<'scope> {
+    key: v8::Local<'scope, v8::String>,
+    location: u32,
+    #[webapi(constructor_default = false)]
+    repeat: bool,
+}
+
+pub(in crate::context_bootstrap) fn keyboard_event_init_callback<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    args: v8::FunctionCallbackArguments<'s>,
+    _rv: v8::ReturnValue<'_, v8::Value>,
+) {
+    let Some(parsed) = webidl::parse_args::<InitKeyboardEventArgs>(scope, &args) else {
+        return;
+    };
+    let event = event_backing(scope, args.this());
+    if !reinitialize_event_object(
+        scope,
+        event,
+        &parsed.event_type.0,
+        parsed.bubbles,
+        parsed.cancelable,
+    ) {
+        return;
+    }
+    initialize_legacy_ui_event(scope, event, parsed.view, 0);
+    let key = v8_string_from_utf16_units(scope, &parsed.key.0).expect("KeyboardEvent key");
+    super::super::modifiers::initialize_legacy_event_modifiers(
+        scope,
+        event,
+        parsed.ctrl_key,
+        parsed.alt_key,
+        parsed.shift_key,
+        parsed.meta_key,
+    );
+    LegacyKeyboardEventInitDeclaration::new(key, parsed.location)
+        .initialize(scope, event)
+        .expect("legacy KeyboardEvent init declaration should initialize");
 }
diff --git a/moli-renderer-v8/src/context_bootstrap/events/ui.rs b/moli-renderer-v8/src/context_bootstrap/events/ui.rs
new file mode 100644
index 0000000000..2c20f71fb3
--- /dev/null
+++ b/moli-renderer-v8/src/context_bootstrap/events/ui.rs
@@ -0,0 +1,87 @@
+use crate::webidl;
+use moli_webapi_declare::WebApiObject;
+
+fn nullable_window_value<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    window: Option<v8::Local<'s, v8::Object>>,
+) -> v8::Local<'s, v8::Value> {
+    window
+        .map(Into::into)
+        .unwrap_or_else(|| v8::null(scope).into())
+}
+
+/// Own UIEventInit members, including legacy which, in dictionary order.
+#[derive(Default, webidl::WebIdlDictionary)]
+#[webidl(prefix = "UIEventInit")]
+struct UiEventInitMembers<'s> {
+    #[webidl(default = 0)]
+    detail: i32,
+    #[webidl(nullable, interface = crate::web_api_interfaces::Window, brand_check = crate::context_bootstrap::is_window_receiver)]
+    view: Option<v8::Local<'s, v8::Object>>,
+    #[webidl(default = 0)]
+    which: u32,
+}
+
+#[derive(Default)]
+pub(super) struct UiEventInit<'s> {
+    flags: (bool, bool, bool),
+    members: UiEventInitMembers<'s>,
+}
+
+impl<'s> webidl::WebIdlDictionary<'s> for UiEventInit<'s> {
+    fn parse_dictionary(
+        scope: &mut v8::PinScope<'s, '_>,
+        object: v8::Local<'s, v8::Object>,
+    ) -> Result<Self, webidl::WebIdlError> {
+        // Read inherited members before own members. Derived dictionaries
+        // delegate here so each observable getter runs once, base-first.
+        Ok(Self {
+            flags: super::init::parse_event_init(scope, Some(object))?,
+            members: webidl::parse_dictionary_object(scope, object)?,
+        })
+    }
+}
+
+impl<'s> UiEventInit<'s> {
+    pub(super) fn event_flags(&self) -> (bool, bool, bool) {
+        self.flags
+    }
+
+    pub(super) fn initialize(
+        self,
+        scope: &mut v8::PinScope<'s, '_>,
+        event: v8::Local<'s, v8::Object>,
+    ) {
+        let view = nullable_window_value(scope, self.members.view);
+        UiEventState::new(view, self.members.detail, self.members.which)
+            .initialize(scope, event)
+            .expect("UIEvent state should initialize");
+    }
+}
+
+#[derive(WebApiObject)]
+#[webapi(plain, data_properties, enumerable)]
+struct UiEventState<'s> {
+    view: v8::Local<'s, v8::Value>,
+    detail: i32,
+    which: u32,
+}
+
+#[derive(WebApiObject)]
+#[webapi(plain, data_properties, enumerable)]
+struct LegacyUiEventState<'s> {
+    view: v8::Local<'s, v8::Value>,
+    detail: i32,
+}
+
+pub(in crate::context_bootstrap) fn initialize_legacy_ui_event<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    event: v8::Local<'s, v8::Object>,
+    view: Option<v8::Local<'s, v8::Object>>,
+    detail: i32,
+) {
+    let view = nullable_window_value(scope, view);
+    LegacyUiEventState::new(view, detail)
+        .initialize(scope, event)
+        .expect("legacy UIEvent state should initialize");
+}
diff --git a/moli-renderer-v8/src/context_bootstrap/events/wrappers.rs b/moli-renderer-v8/src/context_bootstrap/events/wrappers.rs
index 3e57f6ea89..917d2caebc 100644
--- a/moli-renderer-v8/src/context_bootstrap/events/wrappers.rs
+++ b/moli-renderer-v8/src/context_bootstrap/events/wrappers.rs
@@ -248,6 +248,14 @@ pub(in crate::context_bootstrap) fn event_value_attribute_getter<'s>(
     event_attribute_getter::<0>(scope, args, rv);
 }
 
+pub(in crate::context_bootstrap) fn event_platform_attribute_getter<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    args: v8::FunctionCallbackArguments<'s>,
+    rv: v8::ReturnValue<'_, v8::Value>,
+) {
+    event_attribute_getter::<2>(scope, args, rv);
+}
+
 pub(super) fn event_attribute_in_wrapper<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     wrapper: v8::Local<'s, v8::Object>,
diff --git a/moli-renderer-v8/src/script_vm/tests/browser_api/keyboard_event_init.js b/moli-renderer-v8/src/script_vm/tests/browser_api/keyboard_event_init.js
new file mode 100644
index 0000000000..2a007b1960
--- /dev/null
+++ b/moli-renderer-v8/src/script_vm/tests/browser_api/keyboard_event_init.js
@@ -0,0 +1,216 @@
+(() => {
+  const rows = [], observed = {};
+  const check = (name, run) => {
+    try {
+      if (run() !== true) throw new Error('assertion failed');
+      rows.push({name, passed: true});
+    } catch (error) {
+      rows.push({name, passed: false, error: String(error), stack: error.stack});
+    }
+  };
+  const typeError = (run, realm = globalThis) => {
+    try { run(); } catch (error) { return error instanceof realm.TypeError; }
+    return false;
+  };
+  const throws = (run, sentinel) => {
+    try { run(); } catch (error) { return error === sentinel; }
+    return false;
+  };
+  const other = document.getElementById('child').contentWindow;
+  const legacy = (key, location = 0, view = null) => {
+    const event = new KeyboardEvent('before');
+    event.initKeyboardEvent('after', false, false, view, key, location);
+    return event;
+  };
+  const snapshot = event => JSON.stringify([
+    event.type, event.bubbles, event.cancelable, event.composed, event.defaultPrevented,
+    event.detail, event.key, event.code, event.location, event.which, event.charCode,
+    event.keyCode, event.repeat, event.isComposing, event.ctrlKey, event.altKey,
+    event.shiftKey, event.metaKey, event.getModifierState('AltGraph')
+  ]);
+  const texts = ['', '\ud800', '\udc00', '\ud83d\ude00', 'a\ud800b\udc00\u0000\ud83d\ude00'];
+  for (let i = 0; i < texts.length; i++) {
+    const text = texts[i];
+    for (const field of ['key', 'code']) {
+      check(`constructor ${field} UTF-16 ${i}`, () => new KeyboardEvent('x', {[field]: text})[field] === text);
+    }
+    check(`legacy key UTF-16 ${i}`, () => legacy(text).key === text);
+  }
+  for (const [value, expected] of [[undefined, ''], [null, 'null'], [23, '23'], [true, 'true']]) {
+    for (const field of ['key', 'code']) {
+      check(`constructor ${field} ${String(value)}`, () => new KeyboardEvent('x', {[field]: value})[field] === expected);
+    }
+    check(`legacy key ${String(value)}`, () => legacy(value).key === expected);
+  }
+  for (const field of ['key', 'code']) {
+    check(`constructor ${field} string hint once`, () => {
+      let count = 0;
+      const value = {[Symbol.toPrimitive](hint) { if (hint !== 'string') throw new Error(hint); count++; return texts[4]; }};
+      return new KeyboardEvent('x', {[field]: value})[field] === texts[4] && count === 1;
+    });
+    check(`constructor ${field} Symbol rejects`, () => typeError(() => new KeyboardEvent('x', {[field]: Symbol()})));
+  }
+  check('legacy key string hint once', () => {
+    let count = 0;
+    const value = {[Symbol.toPrimitive](hint) { if (hint !== 'string') throw new Error(hint); count++; return texts[4]; }};
+    return legacy(value).key === texts[4] && count === 1;
+  });
+  check('legacy Symbol key rejects', () => typeError(() => legacy(Symbol())));
+  check('constructor default payload', () => {
+    const e = new KeyboardEvent('x');
+    return e.view === null && e.detail === 0 && e.key === '' && e.code === '' && e.location === 0 &&
+      e.charCode === 0 && e.keyCode === 0 && e.which === 0 && !e.repeat && !e.isComposing && !e.isTrusted;
+  });
+  for (const value of [undefined, null]) {
+    check(`nullish dictionary ${String(value)}`, () => new KeyboardEvent('x', value).view === null);
+  }
+  for (const value of [false, 1, '', Symbol(), 1n]) {
+    check(`invalid dictionary ${typeof value}`, () => typeError(() => new KeyboardEvent('x', value)));
+  }
+  for (const [field, value, expected] of [
+    ['detail', 4.9, 4], ['detail', -4.9, -4], ['detail', 2147483648, -2147483648],
+    ['location', -1, 4294967295], ['location', 4294967297, 1], ['location', 1.9, 1],
+    ['charCode', -1, 4294967295], ['keyCode', -1, 4294967295], ['which', -1, 4294967295],
+  ]) {
+    check(`constructor ${field} integer ${value}`, () => new KeyboardEvent('x', {[field]: value})[field] === expected);
+  }
+  for (const field of ['detail', 'location', 'charCode', 'keyCode', 'which']) {
+    for (const value of [NaN, Infinity, -Infinity]) {
+      check(`constructor ${field} nonfinite ${value}`, () => new KeyboardEvent('x', {[field]: value})[field] === 0);
+    }
+    for (const value of [1n, Symbol()]) {
+      check(`constructor ${field} rejects ${typeof value}`, () => typeError(() => new KeyboardEvent('x', {[field]: value})));
+    }
+  }
+  for (const [value, expected] of [[-1, 4294967295], [4294967297, 1], [1.9, 1], [NaN, 0], [Infinity, 0]]) {
+    check(`legacy location integer ${value}`, () => legacy('', value).location === expected);
+  }
+  check('legacy BigInt location rejects', () => typeError(() => legacy('', 1n)));
+  const modifierMembers = ['altKey', 'ctrlKey', 'metaKey', 'modifierAltGraph', 'modifierCapsLock',
+    'modifierFn', 'modifierFnLock', 'modifierHyper', 'modifierNumLock', 'modifierScrollLock',
+    'modifierSuper', 'modifierSymbol', 'modifierSymbolLock', 'shiftKey'];
+  const order = ['bubbles', 'cancelable', 'composed', 'detail', 'view', 'which', ...modifierMembers,
+    'charCode', 'code', 'isComposing', 'key', 'keyCode', 'location', 'repeat'];
+  check('complete inherited dictionary order', () => {
+    const seen = [];
+    new KeyboardEvent('x', new Proxy({}, {get(_target, name) { seen.push(name); }}));
+    observed.dictionaryReads = seen;
+    return seen.join() === order.join();
+  });
+  for (const stop of order) {
+    check(`dictionary getter exception stops at ${stop}`, () => {
+      const sentinel = {};
+      const seen = [];
+      const init = new Proxy({}, {get(_target, name) { seen.push(name); if (name === stop) throw sentinel; }});
+      return throws(() => new KeyboardEvent('x', init), sentinel) && seen.join() === order.slice(0, order.indexOf(stop) + 1).join();
+    });
+  }
+  for (const field of ['detail', 'which', 'code', 'key', 'charCode', 'keyCode', 'location']) {
+    check(`constructor ${field} conversion exception stops`, () => {
+      const sentinel = {}, seen = [];
+      const init = new Proxy({}, {get(_target, name) {
+        seen.push(name);
+        if (name === field) return {[Symbol.toPrimitive]() { throw sentinel; }};
+      }});
+      return throws(() => new KeyboardEvent('x', init), sentinel) && seen.join() === order.slice(0, order.indexOf(field) + 1).join();
+    });
+  }
+  check('constructor type converts before dictionary', () => {
+    const seen = [];
+    const type = {toString() {seen.push('type'); return texts[4];}};
+    const e = new KeyboardEvent(type, new Proxy({}, {get(_target, name) {seen.push(name);}}));
+    return e.type === texts[4] && seen.join() === ['type', ...order].join();
+  });
+  check('constructor inherits dictionary members', () => {
+    const e = new KeyboardEvent('x', Object.create({key: texts[4], code: texts[3], location: -1, view: other, ctrlKey: true}));
+    return e.key === texts[4] && e.code === texts[3] && e.location === 4294967295 && e.view === other && e.getModifierState('Control');
+  });
+  const views = [['window', window], ['child', other], ['null', null], ['undefined', undefined]];
+  for (const [name, view] of views) {
+    const expected = view === undefined ? null : view;
+    check(`constructor accepts ${name} view`, () => new KeyboardEvent('x', {view}).view === expected);
+    check(`legacy accepts ${name} view`, () => legacy('', 0, view).view === expected);
+  }
+  let traps = 0;
+  const handler = {get() {traps++; throw new Error('get trap');}, getPrototypeOf() {traps++; throw new Error('prototype trap');}};
+  const revoked = Proxy.revocable(window, handler); revoked.revoke();
+  for (const [name, view] of [['ordinary', {}], ['prototype', Object.create(window)], ['proxy', new Proxy(window, handler)], ['revoked', revoked.proxy], ['number', 1]]) {
+    check(`constructor rejects ${name} view before later conversion`, () => {
+      let converted = false;
+      const result = typeError(() => new KeyboardEvent('x', {view, code: {toString() {converted = true; return '';}}}));
+      return result && !converted && traps === 0;
+    });
+    check(`legacy rejects ${name} view before key conversion`, () => {
+      let converted = false;
+      const result = typeError(() => legacy({toString() {converted = true; return '';}}, 0, view));
+      return result && !converted && traps === 0;
+    });
+  }
+  check('legacy method descriptor', () => {
+    const d = Object.getOwnPropertyDescriptor(KeyboardEvent.prototype, 'initKeyboardEvent');
+    return d.value.name === 'initKeyboardEvent' && d.value.length === 1 && d.enumerable && d.configurable && d.writable;
+  });
+  check('legacy missing required type', () => typeError(() => new KeyboardEvent('x').initKeyboardEvent()));
+  check('legacy modifier booleans do not invoke author conversion', () => {
+    const e = new KeyboardEvent('x', {modifierAltGraph: true, modifierCapsLock: true});
+    const truthy = {[Symbol.toPrimitive]() {throw new Error('boolean conversion');}};
+    e.initKeyboardEvent('y', false, false, null, '', 0, truthy, false, truthy, false);
+    return e.ctrlKey && !e.altKey && e.shiftKey && !e.metaKey && e.getModifierState('Control') &&
+      e.getModifierState('Shift') && !e.getModifierState('AltGraph') && !e.getModifierState('CapsLock');
+  });
+  check('legacy payload defaults preserve unrelated code and composing state', () => {
+    const e = new KeyboardEvent('before', {code:'KeyB', isComposing:true, repeat:true, composed:true, key:'old', view:window, detail:7, keyCode:8, charCode:9, which:10});
+    const result = e.initKeyboardEvent('after');
+    return result === undefined && e.type === 'after' && e.view === null && e.detail === 0 && e.key === '' &&
+      e.code === 'KeyB' && e.isComposing && !e.repeat && e.location === 0 && e.keyCode === 8 && e.charCode === 9 && e.which === 10 && e.composed;
+  });
+  check('legacy arguments convert left to right before mutation', () => {
+    const e = new KeyboardEvent('before', {key:'old', location:3});
+    const old = snapshot(e), seen = [], sentinel = {};
+    const key = {toString() {seen.push('key'); return 'new';}};
+    const location = {valueOf() {seen.push('location'); throw sentinel;}};
+    return throws(() => e.initKeyboardEvent({toString() {seen.push('type'); return 'new';}}, true, true, null, key, location), sentinel) &&
+      seen.join() === 'type,key,location' && snapshot(e) === old;
+  });
+  check('legacy ignored extra arguments are not converted', () => {
+    const e = new KeyboardEvent('before');
+    e.initKeyboardEvent('after', false, false, null, '', 0, false, false, false, false, {[Symbol.toPrimitive]() {throw new Error('extra conversion');}});
+    return e.type === 'after';
+  });
+  check('legacy dispatch guard follows conversions and preserves state', () => {
+    const e = new KeyboardEvent('before', {key:'old', ctrlKey:true, code:'KeyB', composed:true});
+    const before = snapshot(e), target = new EventTarget();
+    let converted = false, ignored = false, propagated = false;
+    target.addEventListener('before', () => {
+      e.initKeyboardEvent('after', false, false, null, {toString() {converted = true; return 'new';}}, 1, false, true);
+      ignored = snapshot(e) === before;
+      propagated = typeError(() => e.initKeyboardEvent('after', false, false, null, Symbol()));
+    });
+    return target.dispatchEvent(e) && converted && ignored && propagated && snapshot(e) === before;
+  });
+  const real = new KeyboardEvent('before');
+  const revokedEvent = Proxy.revocable(real, handler); revokedEvent.revoke();
+  for (const [name, receiver] of [['ordinary', {}], ['prototype', Object.create(KeyboardEvent.prototype)], ['inherit-real', Object.create(real)], ['proxy', new Proxy(real, handler)], ['revoked', revokedEvent.proxy], ['other-interface', new MouseEvent('x')]]) {
+    check(`legacy rejects ${name} receiver before conversion`, () => {
+      let converted = false;
+      const type = {toString() {converted = true; return 'x';}};
+      return typeError(() => KeyboardEvent.prototype.initKeyboardEvent.call(receiver, type)) && !converted && traps === 0;
+    });
+  }
+  check('cross-realm genuine receiver and Window view', () => {
+    const e = new other.KeyboardEvent('before');
+    KeyboardEvent.prototype.initKeyboardEvent.call(e, 'after', true, false, window, texts[4], -1, true);
+    return e.type === 'after' && e.key === texts[4] && e.view === window && e.location === 4294967295 && e.ctrlKey && e instanceof other.KeyboardEvent;
+  });
+  check('cross-realm method TypeError uses callee realm', () => typeError(() => other.KeyboardEvent.prototype.initKeyboardEvent.call({}, 'x'), other));
+  check('cross-realm constructor TypeError uses callee realm', () => typeError(() => new other.KeyboardEvent('x', {view:{}}), other));
+  check('cross-realm argument TypeError uses callee realm', () => typeError(() => other.KeyboardEvent.prototype.initKeyboardEvent.call(real, 'x', false, false, null, Symbol()), other));
+  check('genuine receiver remains valid after prototype replacement', () => {
+    const e = new KeyboardEvent('before');
+    Object.setPrototypeOf(e, null);
+    KeyboardEvent.prototype.initKeyboardEvent.call(e, 'after');
+    return Object.getOwnPropertyDescriptor(Event.prototype, 'type').get.call(e) === 'after';
+  });
+  globalThis.__keyboardEventResults = {rows, observed, total: rows.length, passed: rows.filter(row => row.passed).length};
+  return rows.every(row => row.passed);
+})()
diff --git a/moli-renderer-v8/src/script_vm/tests/browser_api/keyboard_event_init.rs b/moli-renderer-v8/src/script_vm/tests/browser_api/keyboard_event_init.rs
new file mode 100644
index 0000000000..712a2b07eb
--- /dev/null
+++ b/moli-renderer-v8/src/script_vm/tests/browser_api/keyboard_event_init.rs
@@ -0,0 +1,16 @@
+use super::*;
+
+#[test]
+fn keyboard_event_initializers_preserve_dom_strings_and_convert_complete_dictionaries() {
+    let mut vm = new_storage_html_test_vm("https://keyboard-event-init-webidl.test/");
+    vm.eval("document.body.innerHTML = '<iframe id=child></iframe>';")
+        .unwrap();
+    let result = vm.eval(include_str!("keyboard_event_init.js")).unwrap();
+    assert_eq!(
+        result,
+        "true",
+        "{}",
+        vm.eval("JSON.stringify(__keyboardEventResults.rows.filter(row=>!row.passed))")
+            .unwrap()
+    );
+}
diff --git a/moli-renderer-v8/src/script_vm/tests/browser_api/mod.rs b/moli-renderer-v8/src/script_vm/tests/browser_api/mod.rs
index 91a315bc2e..b7ada18391 100644
--- a/moli-renderer-v8/src/script_vm/tests/browser_api/mod.rs
+++ b/moli-renderer-v8/src/script_vm/tests/browser_api/mod.rs
@@ -72,6 +72,7 @@ mod document_domain_lifetime;
 mod document_domain_setter;
 
 mod error_event_init;
+mod keyboard_event_init;
 mod message_event_init;
 mod window_current_event_private;