diff --git a/src/browser/js/bridge.zig b/src/browser/js/bridge.zig
index 850c8070..68f5e489 100644
--- a/src/browser/js/bridge.zig
+++ b/src/browser/js/bridge.zig
@@ -550,6 +550,7 @@ pub const JsApis = flattenTypes(&.{
     @import("../webapi/Event.zig"),
     @import("../webapi/event/CustomEvent.zig"),
     @import("../webapi/event/ErrorEvent.zig"),
+    @import("../webapi/event/MessageEvent.zig"),
     @import("../webapi/event/ProgressEvent.zig"),
     @import("../webapi/EventTarget.zig"),
     @import("../webapi/Location.zig"),
diff --git a/src/browser/tests/custom_elements/attribute_changed.html b/src/browser/tests/custom_elements/attribute_changed.html
index f94de7f3..2ebbf99e 100644
--- a/src/browser/tests/custom_elements/attribute_changed.html
+++ b/src/browser/tests/custom_elements/attribute_changed.html
@@ -122,6 +122,10 @@
     testing.expectEqual(0, callbackCalls.length);
 
     customElements.define('upgrade-attr-element', UpgradeAttrElement);
+    testing.expectEqual(0, callbackCalls.length);
+
+    document.body.appendChild(el);
+
     testing.expectEqual(1, callbackCalls.length);
     testing.expectEqual('existing', callbackCalls[0].name);
     testing.expectEqual(null, callbackCalls[0].oldValue);
diff --git a/src/browser/tests/custom_elements/upgrade.html b/src/browser/tests/custom_elements/upgrade.html
index 73f1367c..3827ca82 100644
--- a/src/browser/tests/custom_elements/upgrade.html
+++ b/src/browser/tests/custom_elements/upgrade.html
@@ -181,4 +181,175 @@
     testing.expectEqual(null, attributeChangedCalls[1].oldValue);
     testing.expectEqual('world', attributeChangedCalls[1].newValue);
 }
+
+{
+    let attributeChangedCalls = [];
+    let connectedCalls = 0;
+
+    class DetachedWithAttrs extends HTMLElement {
+        static get observedAttributes() {
+            return ['foo'];
+        }
+
+        attributeChangedCallback(name, oldValue, newValue) {
+            attributeChangedCalls.push({ name, oldValue, newValue });
+        }
+
+        connectedCallback() {
+            connectedCalls++;
+        }
+    }
+
+    const container = document.createElement('div');
+    container.innerHTML = '<detached-with-attrs foo="bar"></detached-with-attrs>';
+
+    testing.expectEqual(0, attributeChangedCalls.length);
+
+    customElements.define('detached-with-attrs', DetachedWithAttrs);
+
+    testing.expectEqual(0, attributeChangedCalls.length);
+    testing.expectEqual(0, connectedCalls);
+
+    document.body.appendChild(container);
+
+    testing.expectEqual(1, attributeChangedCalls.length);
+    testing.expectEqual('foo', attributeChangedCalls[0].name);
+    testing.expectEqual(null, attributeChangedCalls[0].oldValue);
+    testing.expectEqual('bar', attributeChangedCalls[0].newValue);
+    testing.expectEqual(1, connectedCalls);
+}
+
+{
+    let attributeChangedCalls = [];
+    let constructorCalled = 0;
+
+    class ManualUpgradeWithAttrs extends HTMLElement {
+        static get observedAttributes() {
+            return ['x', 'y'];
+        }
+
+        constructor() {
+            super();
+            constructorCalled++;
+        }
+
+        attributeChangedCallback(name, oldValue, newValue) {
+            attributeChangedCalls.push({ name, oldValue, newValue });
+        }
+    }
+
+    customElements.define('manual-upgrade-with-attrs', ManualUpgradeWithAttrs);
+
+    const container = document.createElement('div');
+    container.innerHTML = '<manual-upgrade-with-attrs x="1" y="2"></manual-upgrade-with-attrs>';
+
+    testing.expectEqual(1, constructorCalled);
+    testing.expectEqual(2, attributeChangedCalls.length);
+
+    const elem = container.querySelector('manual-upgrade-with-attrs');
+    elem.setAttribute('z', '3');
+
+    customElements.upgrade(container);
+
+    testing.expectEqual(1, constructorCalled);
+    testing.expectEqual(2, attributeChangedCalls.length);
+}
+
+{
+    let attributeChangedCalls = [];
+
+    class MixedAttrs extends HTMLElement {
+        static get observedAttributes() {
+            return ['watched'];
+        }
+
+        attributeChangedCallback(name, oldValue, newValue) {
+            attributeChangedCalls.push({ name, oldValue, newValue });
+        }
+    }
+
+    const container = document.createElement('div');
+    container.innerHTML = '<mixed-attrs watched="yes" ignored="no" also-ignored="maybe"></mixed-attrs>';
+    document.body.appendChild(container);
+
+    testing.expectEqual(0, attributeChangedCalls.length);
+
+    customElements.define('mixed-attrs', MixedAttrs);
+
+    testing.expectEqual(1, attributeChangedCalls.length);
+    testing.expectEqual('watched', attributeChangedCalls[0].name);
+    testing.expectEqual('yes', attributeChangedCalls[0].newValue);
+}
+
+{
+    let attributeChangedCalls = [];
+
+    class EmptyAttr extends HTMLElement {
+        static get observedAttributes() {
+            return ['empty', 'non-empty'];
+        }
+
+        attributeChangedCallback(name, oldValue, newValue) {
+            attributeChangedCalls.push({ name, oldValue, newValue });
+        }
+    }
+
+    const container = document.createElement('div');
+    container.innerHTML = '<empty-attr empty="" non-empty="value"></empty-attr>';
+    document.body.appendChild(container);
+
+    customElements.define('empty-attr', EmptyAttr);
+
+    testing.expectEqual(2, attributeChangedCalls.length);
+    testing.expectEqual('empty', attributeChangedCalls[0].name);
+    testing.expectEqual('', attributeChangedCalls[0].newValue);
+    testing.expectEqual('non-empty', attributeChangedCalls[1].name);
+    testing.expectEqual('value', attributeChangedCalls[1].newValue);
+}
+
+{
+    let parentCalls = [];
+    let childCalls = [];
+
+    class NestedParent extends HTMLElement {
+        static get observedAttributes() {
+            return ['parent-attr'];
+        }
+
+        attributeChangedCallback(name, oldValue, newValue) {
+            parentCalls.push({ name, oldValue, newValue });
+        }
+    }
+
+    class NestedChild extends HTMLElement {
+        static get observedAttributes() {
+            return ['child-attr'];
+        }
+
+        attributeChangedCallback(name, oldValue, newValue) {
+            childCalls.push({ name, oldValue, newValue });
+        }
+    }
+
+    const container = document.createElement('div');
+    container.innerHTML = '<nested-parent parent-attr="p"><nested-child child-attr="c"></nested-child></nested-parent>';
+    document.body.appendChild(container);
+
+    testing.expectEqual(0, parentCalls.length);
+    testing.expectEqual(0, childCalls.length);
+
+    customElements.define('nested-parent', NestedParent);
+
+    testing.expectEqual(1, parentCalls.length);
+    testing.expectEqual('parent-attr', parentCalls[0].name);
+    testing.expectEqual('p', parentCalls[0].newValue);
+    testing.expectEqual(0, childCalls.length);
+
+    customElements.define('nested-child', NestedChild);
+
+    testing.expectEqual(1, parentCalls.length);
+    testing.expectEqual(1, childCalls.length);
+    testing.expectEqual('child-attr', childCalls[0].name);
+    testing.expectEqual('c', childCalls[0].newValue);
+}
 </script>
diff --git a/src/browser/tests/event/message.html b/src/browser/tests/event/message.html
new file mode 100644
index 00000000..079f9c7a
--- /dev/null
+++ b/src/browser/tests/event/message.html
@@ -0,0 +1,170 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+
+<script id=messageEventConstructor>
+{
+  const evt = new MessageEvent('message', {
+    data: { foo: 'bar', count: 42 },
+    origin: 'https://example.com',
+    source: window
+  });
+
+  testing.expectEqual('message', evt.type);
+  testing.expectEqual({ foo: 'bar', count: 42 }, evt.data);
+  testing.expectEqual('https://example.com', evt.origin);
+  testing.expectEqual(window, evt.source);
+  testing.expectEqual(false, evt.bubbles);
+  testing.expectEqual(false, evt.cancelable);
+}
+</script>
+
+<script id=messageEventWithString>
+{
+  const evt2 = new MessageEvent('message', {
+    data: 'Hello, World!',
+    origin: 'https://test.com'
+  });
+
+  testing.expectEqual('Hello, World!', evt2.data);
+  testing.expectEqual('https://test.com', evt2.origin);
+}
+</script>
+
+<script id=messageEventDefaults>
+{
+  const evt3 = new MessageEvent('message');
+
+  testing.expectEqual('', evt3.origin);
+  testing.expectEqual(false, evt3.bubbles);
+  testing.expectEqual(false, evt3.cancelable);
+}
+</script>
+
+<script id=messageEventInheritance>
+{
+  const evt4 = new MessageEvent('custom');
+
+  testing.expectEqual('custom', evt4.type);
+  testing.expectEqual(Event.NONE, evt4.eventPhase);
+  testing.expectEqual(false, evt4.defaultPrevented);
+}
+</script>
+
+<script id=postMessageBasic>
+{
+  let receivedEvent = null;
+
+  const handler = (e) => {
+    receivedEvent = e;
+  };
+  window.addEventListener('message', handler, { once: true });
+
+  window.postMessage('test data', '*');
+
+  testing.eventually(() => {
+    testing.expectEqual('test data', receivedEvent.data);
+    testing.expectEqual(window, receivedEvent.source);
+    testing.expectEqual('message', receivedEvent.type);
+  });
+}
+</script>
+
+<script id=postMessageWithObject>
+{
+  let receivedData = null;
+
+  const handler = (e) => {
+    receivedData = e.data;
+  };
+  window.addEventListener('message', handler, { once: true });
+
+  const testObj = { type: 'test', value: 123, nested: { key: 'value' } };
+  window.postMessage(testObj, '*');
+
+  testing.eventually(() => {
+    testing.expectEqual(testObj, receivedData);
+  });
+}
+</script>
+
+<script id=messageEventWithBubbles>
+{
+  const evt = new MessageEvent('message', {
+    data: 'test',
+    bubbles: true,
+    cancelable: true
+  });
+
+  testing.expectEqual(true, evt.bubbles);
+  testing.expectEqual(true, evt.cancelable);
+}
+</script>
+
+<script id=postMessageWithNumber>
+{
+  let received = null;
+
+  const handler = (e) => {
+    received = e.data;
+  };
+  window.addEventListener('message', handler, { once: true });
+
+  window.postMessage(42, '*');
+
+  testing.eventually(() => {
+    testing.expectEqual(42, received);
+  });
+}
+</script>
+
+<script id=postMessageWithArray>
+{
+  let received = null;
+
+  const handler = (e) => {
+    received = e.data;
+  };
+  window.addEventListener('message', handler, { once: true });
+
+  const arr = [1, 2, 3, 'test'];
+  window.postMessage(arr, '*');
+
+  testing.eventually(() => {
+    testing.expectEqual(arr, received);
+  });
+}
+</script>
+
+<script id=postMessageWithNull>
+{
+  let received = undefined;
+
+  const handler = (e) => {
+    received = e.data;
+  };
+  window.addEventListener('message', handler, { once: true });
+
+  window.postMessage(null, '*');
+
+  testing.eventually(() => {
+    testing.expectEqual(null, received);
+  });
+}
+</script>
+
+<script id=messageEventOriginFromLocation>
+{
+  let receivedOrigin = null;
+
+  const handler = (e) => {
+    receivedOrigin = e.origin;
+  };
+  window.addEventListener('message', handler, { once: true });
+
+  window.postMessage('test', '*');
+
+  testing.eventually(() => {
+    testing.expectEqual('http://127.0.0.1:9582', receivedOrigin);
+  });
+}
+</script>
diff --git a/src/browser/tests/event/message_multiple_listeners.html b/src/browser/tests/event/message_multiple_listeners.html
new file mode 100644
index 00000000..36f13eb2
--- /dev/null
+++ b/src/browser/tests/event/message_multiple_listeners.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+
+<script id=postMessageMultipleListeners>
+{
+  let count = 0;
+
+  const handler1 = () => { count++; };
+  const handler2 = () => { count++; };
+  window.addEventListener('message', handler1, { once: true });
+  window.addEventListener('message', handler2, { once: true });
+
+  window.postMessage('trigger', '*');
+
+  testing.eventually(() => {
+    testing.expectEqual(2, count);
+  });
+}
+</script>
diff --git a/src/browser/webapi/CustomElementRegistry.zig b/src/browser/webapi/CustomElementRegistry.zig
index 318b80e6..6fc67621 100644
--- a/src/browser/webapi/CustomElementRegistry.zig
+++ b/src/browser/webapi/CustomElementRegistry.zig
@@ -92,6 +92,11 @@ pub fn define(self: *CustomElementRegistry, name: []const u8, constructor: js.Fu
             continue;
         }
 
+        if (!custom.asElement().asNode().isConnected()) {
+            idx += 1;
+            continue;
+        }
+
         upgradeCustomElement(custom, definition, page) catch {
             _ = page._undefined_custom_elements.swapRemove(idx);
             continue;
@@ -134,7 +139,7 @@ fn upgradeElement(self: *CustomElementRegistry, element: *Element, page: *Page)
     try upgradeCustomElement(custom, definition, page);
 }
 
-fn upgradeCustomElement(custom: *Custom, definition: *CustomElementDefinition, page: *Page) !void {
+pub fn upgradeCustomElement(custom: *Custom, definition: *CustomElementDefinition, page: *Page) !void {
     custom._definition = definition;
 
     // Reset callback flags since this is a fresh upgrade
diff --git a/src/browser/webapi/Event.zig b/src/browser/webapi/Event.zig
index b11a83fa..56461eb5 100644
--- a/src/browser/webapi/Event.zig
+++ b/src/browser/webapi/Event.zig
@@ -49,9 +49,10 @@ pub const EventPhase = enum(u8) {
 
 pub const Type = union(enum) {
     generic,
-    progress_event: *@import("event/ProgressEvent.zig"),
     error_event: *@import("event/ErrorEvent.zig"),
     custom_event: *@import("event/CustomEvent.zig"),
+    message_event: *@import("event/MessageEvent.zig"),
+    progress_event: *@import("event/ProgressEvent.zig"),
 };
 
 const Options = struct {
diff --git a/src/browser/webapi/Window.zig b/src/browser/webapi/Window.zig
index f025d1ed..ba3683b6 100644
--- a/src/browser/webapi/Window.zig
+++ b/src/browser/webapi/Window.zig
@@ -34,6 +34,7 @@ const Location = @import("Location.zig");
 const Fetch = @import("net/Fetch.zig");
 const EventTarget = @import("EventTarget.zig");
 const ErrorEvent = @import("event/ErrorEvent.zig");
+const MessageEvent = @import("event/MessageEvent.zig");
 const MediaQueryList = @import("css/MediaQueryList.zig");
 const storage = @import("storage/storage.zig");
 const Element = @import("Element.zig");
@@ -255,6 +256,28 @@ pub fn getComputedStyle(_: *const Window, _: *Element, page: *Page) !*CSSStyleDe
     return CSSStyleDeclaration.init(null, page);
 }
 
+pub fn postMessage(self: *Window, message: js.Object, target_origin: ?[]const u8, page: *Page) !void {
+    // For now, we ignore targetOrigin checking and just dispatch the message
+    // In a full implementation, we would validate the origin
+    _ = target_origin;
+
+    // postMessage queues a task (not a microtask), so use the scheduler
+    const origin = try self._location.getOrigin(page);
+    const callback = try page._factory.create(PostMessageCallback{
+        .window = self,
+        .message = try message.persist() ,
+        .origin = try page.arena.dupe(u8, origin),
+        .page = page,
+    });
+    errdefer page._factory.destroy(callback);
+
+
+    try page.scheduler.add(callback, PostMessageCallback.run, 0, .{
+        .name = "postMessage",
+        .low_priority = false,
+    });
+}
+
 pub fn btoa(_: *const Window, input: []const u8, page: *Page) ![]const u8 {
     const encoded_len = std.base64.standard.Encoder.calcSize(input.len);
     const encoded = try page.call_arena.alloc(u8, encoded_len);
@@ -268,6 +291,26 @@ pub fn atob(_: *const Window, input: []const u8, page: *Page) ![]const u8 {
     return decoded;
 }
 
+pub fn getLength(_: *const Window) u32 {
+    return 0;
+}
+
+pub fn getInnerWidth(_: *const Window) u32 {
+    return 1920;
+}
+
+pub fn getInnerHeight(_: *const Window) u32 {
+    return 1080;
+}
+
+pub fn getScrollX(_: *const Window) u32 {
+    return 0;
+}
+
+pub fn getScrollY(_: *const Window) u32 {
+    return 0;
+}
+
 const ScheduleOpts = struct {
     repeat: bool,
     params: []js.Object,
@@ -376,6 +419,35 @@ const ScheduleCallback = struct {
     }
 };
 
+const PostMessageCallback = struct {
+    window: *Window,
+    message: js.Object,
+    origin: []const u8,
+    page: *Page,
+
+    fn deinit(self: *PostMessageCallback) void {
+        self.page._factory.destroy(self);
+    }
+
+    fn run(ctx: *anyopaque) !?u32 {
+        const self: *PostMessageCallback = @ptrCast(@alignCast(ctx));
+        defer self.deinit();
+
+        const message_event = try MessageEvent.init("message", .{
+            .data = self.message,
+            .origin = self.origin,
+            .source = self.window,
+            .bubbles = false,
+            .cancelable = false,
+        }, self.page);
+
+        const event = message_event.asEvent();
+        try self.page._event_manager.dispatch(self.window.asEventTarget(), event);
+
+        return null;
+    }
+};
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(Window);
 
@@ -415,27 +487,19 @@ pub const JsApi = struct {
     pub const requestAnimationFrame = bridge.function(Window.requestAnimationFrame, .{});
     pub const cancelAnimationFrame = bridge.function(Window.cancelAnimationFrame, .{});
     pub const matchMedia = bridge.function(Window.matchMedia, .{});
+    pub const postMessage = bridge.function(Window.postMessage, .{});
     pub const btoa = bridge.function(Window.btoa, .{});
     pub const atob = bridge.function(Window.atob, .{});
     pub const reportError = bridge.function(Window.reportError, .{});
     pub const frames = bridge.accessor(Window.getWindow, null, .{ .cache = "frames" });
-    pub const length = bridge.accessor(struct {
-        fn wrap(_: *const Window) u32 {
-            return 0;
-        }
-    }.wrap, null, .{ .cache = "length" });
-
-    pub const innerWidth = bridge.accessor(struct {
-        fn wrap(_: *const Window) u32 {
-            return 1920;
-        }
-    }.wrap, null, .{ .cache = "innerWidth" });
-    pub const innerHeight = bridge.accessor(struct {
-        fn wrap(_: *const Window) u32 {
-            return 1080;
-        }
-    }.wrap, null, .{ .cache = "innerHeight" });
     pub const getComputedStyle = bridge.function(Window.getComputedStyle, .{});
+    pub const length = bridge.accessor(Window.getLength, null, .{ .cache = "length" });
+    pub const innerWidth = bridge.accessor(Window.getInnerWidth, null, .{ .cache = "innerWidth" });
+    pub const innerHeight = bridge.accessor(Window.getInnerHeight, null, .{ .cache = "innerHeight" });
+    pub const scrollX = bridge.accessor(Window.getScrollX, null, .{ .cache = "scrollX" });
+    pub const scrollY = bridge.accessor(Window.getScrollY, null, .{ .cache = "scrollY" });
+    pub const pageXOffset = bridge.accessor(Window.getScrollX, null, .{ .cache = "pageXOffset" });
+    pub const pageYOffset = bridge.accessor(Window.getScrollY, null, .{ .cache = "pageYOffset" });
 };
 
 const testing = @import("../../testing.zig");
diff --git a/src/browser/webapi/element/html/Custom.zig b/src/browser/webapi/element/html/Custom.zig
index a8c95d5c..3fc9071f 100644
--- a/src/browser/webapi/element/html/Custom.zig
+++ b/src/browser/webapi/element/html/Custom.zig
@@ -73,6 +73,16 @@ pub fn invokeAttributeChangedCallback(self: *Custom, name: []const u8, old_value
 pub fn invokeConnectedCallbackOnElement(comptime from_parser: bool, element: *Element, page: *Page) !void {
     // Autonomous custom element
     if (element.is(Custom)) |custom| {
+        // If the element is undefined, check if a definition now exists and upgrade
+        if (custom._definition == null) {
+            const name = custom._tag_name.str();
+            if (page.window._custom_elements._definitions.get(name)) |definition| {
+                const CustomElementRegistry = @import("../../CustomElementRegistry.zig");
+                CustomElementRegistry.upgradeCustomElement(custom, definition, page) catch {};
+                return;
+            }
+        }
+
         if (comptime from_parser) {
             // From parser, we know the element is brand new
             custom._connected_callback_invoked = true;
diff --git a/src/browser/webapi/event/MessageEvent.zig b/src/browser/webapi/event/MessageEvent.zig
new file mode 100644
index 00000000..ed59bb2f
--- /dev/null
+++ b/src/browser/webapi/event/MessageEvent.zig
@@ -0,0 +1,90 @@
+// Copyright (C) 2023-2025  Lightpanda (Selecy SAS)
+//
+// Francis Bouvier <francis@lightpanda.io>
+// Pierre Tachoire <pierre@lightpanda.io>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+const js = @import("../../js/js.zig");
+
+const Page = @import("../../Page.zig");
+const Event = @import("../Event.zig");
+const Window = @import("../Window.zig");
+
+const MessageEvent = @This();
+
+_proto: *Event,
+_data: ?js.Object = null,
+_origin: []const u8 = "",
+_source: ?*Window = null,
+
+pub const InitOptions = struct {
+    data: ?js.Object = null,
+    origin: ?[]const u8 = null,
+    source: ?*Window = null,
+    bubbles: bool = false,
+    cancelable: bool = false,
+};
+
+pub fn init(typ: []const u8, opts_: ?InitOptions, page: *Page) !*MessageEvent {
+    const opts = opts_ orelse InitOptions{};
+
+    const event = try page._factory.event(typ, MessageEvent{
+        ._proto = undefined,
+        ._data = if (opts.data) |d| try d.persist() else null,
+        ._origin = if (opts.origin) |str| try page.arena.dupe(u8, str) else "",
+        ._source = opts.source,
+    });
+
+    event._proto._bubbles = opts.bubbles;
+    event._proto._cancelable = opts.cancelable;
+
+    return event;
+}
+
+pub fn asEvent(self: *MessageEvent) *Event {
+    return self._proto;
+}
+
+pub fn getData(self: *const MessageEvent) ?js.Object {
+    return self._data;
+}
+
+pub fn getOrigin(self: *const MessageEvent) []const u8 {
+    return self._origin;
+}
+
+pub fn getSource(self: *const MessageEvent) ?*Window {
+    return self._source;
+}
+
+pub const JsApi = struct {
+    pub const bridge = js.Bridge(MessageEvent);
+
+    pub const Meta = struct {
+        pub const name = "MessageEvent";
+        pub const prototype_chain = bridge.prototypeChain();
+        pub var class_id: bridge.ClassId = undefined;
+    };
+
+    pub const constructor = bridge.constructor(MessageEvent.init, .{});
+    pub const data = bridge.accessor(MessageEvent.getData, null, .{});
+    pub const origin = bridge.accessor(MessageEvent.getOrigin, null, .{});
+    pub const source = bridge.accessor(MessageEvent.getSource, null, .{});
+};
+
+const testing = @import("../../../testing.zig");
+test "WebApi: MessageEvent" {
+    try testing.htmlRunner("event/message.html", .{});
+}