diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index 9ca2b758bff1..80214a67a992 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -1279,8 +1279,13 @@ where
                     warn!("constellation got set final url message for dead pipeline");
                 }
             },
-            FromScriptMsg::PostMessage(browsing_context_id, origin, data) => {
-                self.handle_post_message_msg(browsing_context_id, origin, data);
+            FromScriptMsg::PostMessage {
+                target: browsing_context_id,
+                source: source_pipeline_id,
+                target_origin: origin,
+                data,
+            } => {
+                self.handle_post_message_msg(browsing_context_id, source_pipeline_id, origin, data);
             },
             FromScriptMsg::Focus => {
                 self.handle_focus_msg(source_pipeline_id);
@@ -2844,6 +2849,7 @@ where
     fn handle_post_message_msg(
         &mut self,
         browsing_context_id: BrowsingContextId,
+        source_pipeline: PipelineId,
         origin: Option<ImmutableOrigin>,
         data: Vec<u8>,
     ) {
@@ -2856,7 +2862,17 @@ where
             },
             Some(browsing_context) => browsing_context.pipeline_id,
         };
-        let msg = ConstellationControlMsg::PostMessage(pipeline_id, origin, data);
+        let source_browsing_context = match self.pipelines.get(&source_pipeline) {
+            Some(pipeline) => pipeline.top_level_browsing_context_id,
+            None => return warn!("PostMessage from closed pipeline {:?}", source_pipeline),
+        };
+        let msg = ConstellationControlMsg::PostMessage {
+            target: pipeline_id,
+            source: source_pipeline,
+            source_browsing_context: source_browsing_context,
+            target_origin: origin,
+            data,
+        };
         let result = match self.pipelines.get(&pipeline_id) {
             Some(pipeline) => pipeline.event_loop.send(msg),
             None => return warn!("postMessage to closed pipeline {}.", pipeline_id),
diff --git a/components/script/dom/dedicatedworkerglobalscope.rs b/components/script/dom/dedicatedworkerglobalscope.rs
index edcb07cd4a19..92e26bad3533 100644
--- a/components/script/dom/dedicatedworkerglobalscope.rs
+++ b/components/script/dom/dedicatedworkerglobalscope.rs
@@ -424,7 +424,7 @@ impl DedicatedWorkerGlobalScope {
                     JSAutoCompartment::new(scope.get_cx(), scope.reflector().get_jsobject().get());
                 rooted!(in(scope.get_cx()) let mut message = UndefinedValue());
                 data.read(scope.upcast(), message.handle_mut());
-                MessageEvent::dispatch_jsval(target, scope.upcast(), message.handle(), None);
+                MessageEvent::dispatch_jsval(target, scope.upcast(), message.handle(), None, None);
             },
             WorkerScriptMsg::Common(msg) => {
                 self.upcast::<WorkerGlobalScope>().process_event(msg);
diff --git a/components/script/dom/dissimilaroriginwindow.rs b/components/script/dom/dissimilaroriginwindow.rs
index 662b65b8ef53..4814862881a5 100644
--- a/components/script/dom/dissimilaroriginwindow.rs
+++ b/components/script/dom/dissimilaroriginwindow.rs
@@ -203,11 +203,12 @@ impl DissimilarOriginWindow {
             None => return warn!("postMessage called with no incumbent global"),
             Some(incumbent) => incumbent,
         };
-        let msg = ScriptMsg::PostMessage(
-            self.window_proxy.browsing_context_id(),
-            origin,
-            data.move_to_arraybuffer(),
-        );
+        let msg = ScriptMsg::PostMessage {
+            target: self.window_proxy.browsing_context_id(),
+            source: incumbent.pipeline_id(),
+            target_origin: origin,
+            data: data.move_to_arraybuffer(),
+        };
         let _ = incumbent.script_to_constellation_chan().send(msg);
     }
 }
diff --git a/components/script/dom/eventsource.rs b/components/script/dom/eventsource.rs
index 10c912fb8e4c..5daaf3a5f7b0 100644
--- a/components/script/dom/eventsource.rs
+++ b/components/script/dom/eventsource.rs
@@ -238,6 +238,7 @@ impl EventSourceContext {
                 false,
                 data.handle(),
                 DOMString::from(self.origin.clone()),
+                None,
                 event_source.last_event_id.borrow().clone(),
             )
         };
diff --git a/components/script/dom/messageevent.rs b/components/script/dom/messageevent.rs
index 65c8203913a7..edde48f95e63 100644
--- a/components/script/dom/messageevent.rs
+++ b/components/script/dom/messageevent.rs
@@ -7,24 +7,27 @@ use crate::dom::bindings::codegen::Bindings::MessageEventBinding;
 use crate::dom::bindings::codegen::Bindings::MessageEventBinding::MessageEventMethods;
 use crate::dom::bindings::error::Fallible;
 use crate::dom::bindings::inheritance::Castable;
-use crate::dom::bindings::reflector::reflect_dom_object;
-use crate::dom::bindings::root::DomRoot;
+use crate::dom::bindings::reflector::{reflect_dom_object, DomObject};
+use crate::dom::bindings::root::{Dom, DomRoot};
 use crate::dom::bindings::str::DOMString;
 use crate::dom::bindings::trace::RootedTraceableBox;
 use crate::dom::event::Event;
 use crate::dom::eventtarget::EventTarget;
 use crate::dom::globalscope::GlobalScope;
+use crate::dom::windowproxy::WindowProxy;
 use dom_struct::dom_struct;
-use js::jsapi::{Heap, JSContext};
+use js::jsapi::{Heap, JSContext, JSObject};
 use js::jsval::JSVal;
 use js::rust::HandleValue;
 use servo_atoms::Atom;
+use std::ptr::NonNull;
 
 #[dom_struct]
 pub struct MessageEvent {
     event: Event,
     data: Heap<JSVal>,
     origin: DOMString,
+    source: Option<Dom<WindowProxy>>,
     lastEventId: DOMString,
 }
 
@@ -34,6 +37,7 @@ impl MessageEvent {
             global,
             HandleValue::undefined(),
             DOMString::new(),
+            None,
             DOMString::new(),
         )
     }
@@ -42,12 +46,14 @@ impl MessageEvent {
         global: &GlobalScope,
         data: HandleValue,
         origin: DOMString,
+        source: Option<&WindowProxy>,
         lastEventId: DOMString,
     ) -> DomRoot<MessageEvent> {
         let ev = Box::new(MessageEvent {
             event: Event::new_inherited(),
             data: Heap::default(),
             origin: origin,
+            source: source.map(Dom::from_ref),
             lastEventId: lastEventId,
         });
         let ev = reflect_dom_object(ev, global, MessageEventBinding::Wrap);
@@ -63,9 +69,10 @@ impl MessageEvent {
         cancelable: bool,
         data: HandleValue,
         origin: DOMString,
+        source: Option<&WindowProxy>,
         lastEventId: DOMString,
     ) -> DomRoot<MessageEvent> {
-        let ev = MessageEvent::new_initialized(global, data, origin, lastEventId);
+        let ev = MessageEvent::new_initialized(global, data, origin, source, lastEventId);
         {
             let event = ev.upcast::<Event>();
             event.init_event(type_, bubbles, cancelable);
@@ -78,6 +85,10 @@ impl MessageEvent {
         type_: DOMString,
         init: RootedTraceableBox<MessageEventBinding::MessageEventInit>,
     ) -> Fallible<DomRoot<MessageEvent>> {
+        let source = init
+            .source
+            .as_ref()
+            .and_then(|inner| inner.as_ref().map(|source| source.window_proxy()));
         let ev = MessageEvent::new(
             global,
             Atom::from(type_),
@@ -85,6 +96,7 @@ impl MessageEvent {
             init.parent.cancelable,
             init.data.handle(),
             init.origin.clone(),
+            source.as_ref().map(|source| &**source),
             init.lastEventId.clone(),
         );
         Ok(ev)
@@ -97,6 +109,7 @@ impl MessageEvent {
         scope: &GlobalScope,
         message: HandleValue,
         origin: Option<&str>,
+        source: Option<&WindowProxy>,
     ) {
         let messageevent = MessageEvent::new(
             scope,
@@ -105,6 +118,7 @@ impl MessageEvent {
             false,
             message,
             DOMString::from(origin.unwrap_or("")),
+            source,
             DOMString::new(),
         );
         messageevent.upcast::<Event>().fire(target);
@@ -123,6 +137,14 @@ impl MessageEventMethods for MessageEvent {
         self.origin.clone()
     }
 
+    // https://html.spec.whatwg.org/multipage/#dom-messageevent-source
+    #[allow(unsafe_code)]
+    unsafe fn GetSource(&self, _cx: *mut JSContext) -> Option<NonNull<JSObject>> {
+        self.source
+            .as_ref()
+            .and_then(|source| NonNull::new(source.reflector().get_jsobject().get()))
+    }
+
     // https://html.spec.whatwg.org/multipage/#dom-messageevent-lasteventid
     fn LastEventId(&self) -> DOMString {
         self.lastEventId.clone()
diff --git a/components/script/dom/webidls/MessageEvent.webidl b/components/script/dom/webidls/MessageEvent.webidl
index 1fa4c924901f..c2ef97320966 100644
--- a/components/script/dom/webidls/MessageEvent.webidl
+++ b/components/script/dom/webidls/MessageEvent.webidl
@@ -8,6 +8,8 @@ interface MessageEvent : Event {
   readonly attribute any data;
   readonly attribute DOMString origin;
   readonly attribute DOMString lastEventId;
+  // FIXME(#22617): WindowProxy is not exposed in Worker globals
+  readonly attribute object? source;
   //readonly attribute (WindowProxy or MessagePort)? source;
   //readonly attribute MessagePort[]? ports;
 };
@@ -17,6 +19,7 @@ dictionary MessageEventInit : EventInit {
   DOMString origin = "";
   DOMString lastEventId = "";
   //DOMString channel;
+  Window? source;
   //(WindowProxy or MessagePort)? source;
   //sequence<MessagePort> ports;
 };
diff --git a/components/script/dom/websocket.rs b/components/script/dom/websocket.rs
index 4344078a0174..d03e168e3a13 100644
--- a/components/script/dom/websocket.rs
+++ b/components/script/dom/websocket.rs
@@ -599,6 +599,7 @@ impl TaskOnce for MessageReceivedTask {
                 &global,
                 message.handle(),
                 Some(&ws.origin().ascii_serialization()),
+                None,
             );
         }
     }
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 2993a95456ad..533db98e05bc 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -859,6 +859,9 @@ impl WindowMethods for Window {
         message: HandleValue,
         origin: DOMString,
     ) -> ErrorResult {
+        let source_global = GlobalScope::incumbent().expect("no incumbent global??");
+        let source = source_global.as_window();
+
         // Step 3-5.
         let origin = match &origin[..] {
             "*" => None,
@@ -878,7 +881,7 @@ impl WindowMethods for Window {
         let data = StructuredCloneData::write(cx, message)?;
 
         // Step 9.
-        self.post_message(origin, data);
+        self.post_message(origin, &*source.window_proxy(), data);
         Ok(())
     }
 
@@ -2194,11 +2197,14 @@ impl Window {
     pub fn post_message(
         &self,
         target_origin: Option<ImmutableOrigin>,
+        source: &WindowProxy,
         serialize_with_transfer_result: StructuredCloneData,
     ) {
         let this = Trusted::new(self);
+        let source = Trusted::new(source);
         let task = task!(post_serialised_message: move || {
             let this = this.root();
+            let source = source.root();
 
             // Step 7.1.
             if let Some(target_origin) = target_origin {
@@ -2226,7 +2232,8 @@ impl Window {
                 this.upcast(),
                 this.upcast(),
                 message_clone.handle(),
-                None
+                None,
+                Some(&*source),
             );
         });
         // FIXME(nox): Why are errors silenced here?
diff --git a/components/script/dom/worker.rs b/components/script/dom/worker.rs
index 7135e969c0ec..aeabbeb26453 100644
--- a/components/script/dom/worker.rs
+++ b/components/script/dom/worker.rs
@@ -140,7 +140,7 @@ impl Worker {
         let _ac = JSAutoCompartment::new(global.get_cx(), target.reflector().get_jsobject().get());
         rooted!(in(global.get_cx()) let mut message = UndefinedValue());
         data.read(&global, message.handle_mut());
-        MessageEvent::dispatch_jsval(target, &global, message.handle(), None);
+        MessageEvent::dispatch_jsval(target, &global, message.handle(), None, None);
     }
 
     pub fn dispatch_simple_error(address: TrustedWorkerAddress) {
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 6114e353d971..9ef684b96c4b 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1419,7 +1419,7 @@ impl ScriptThread {
                     ChangeFrameVisibilityStatus(id, ..) => Some(id),
                     NotifyVisibilityChange(id, ..) => Some(id),
                     Navigate(id, ..) => Some(id),
-                    PostMessage(id, ..) => Some(id),
+                    PostMessage { target: id, .. } => Some(id),
                     UpdatePipelineId(_, _, id, _) => Some(id),
                     UpdateHistoryState(id, ..) => Some(id),
                     RemoveHistoryStates(id, ..) => Some(id),
@@ -1592,9 +1592,19 @@ impl ScriptThread {
                 browsing_context_id,
                 visible,
             ),
-            ConstellationControlMsg::PostMessage(pipeline_id, origin, data) => {
-                self.handle_post_message_msg(pipeline_id, origin, data)
-            },
+            ConstellationControlMsg::PostMessage {
+                target: target_pipeline_id,
+                source: source_pipeline_id,
+                source_browsing_context,
+                target_origin: origin,
+                data,
+            } => self.handle_post_message_msg(
+                target_pipeline_id,
+                source_pipeline_id,
+                source_browsing_context,
+                origin,
+                data,
+            ),
             ConstellationControlMsg::UpdatePipelineId(
                 parent_pipeline_id,
                 browsing_context_id,
@@ -2080,12 +2090,33 @@ impl ScriptThread {
     fn handle_post_message_msg(
         &self,
         pipeline_id: PipelineId,
+        source_pipeline_id: PipelineId,
+        source_browsing_context: TopLevelBrowsingContextId,
         origin: Option<ImmutableOrigin>,
         data: Vec<u8>,
     ) {
         match { self.documents.borrow().find_window(pipeline_id) } {
-            None => return warn!("postMessage after pipeline {} closed.", pipeline_id),
-            Some(window) => window.post_message(origin, StructuredCloneData::Vector(data)),
+            None => return warn!("postMessage after target pipeline {} closed.", pipeline_id),
+            Some(window) => {
+                // FIXME: synchronously talks to constellation.
+                // send the required info as part of postmessage instead.
+                let source = match self.remote_window_proxy(
+                    &*window.global(),
+                    source_browsing_context,
+                    source_pipeline_id,
+                    None,
+                ) {
+                    None => {
+                        return warn!(
+                            "postMessage after source pipeline {} closed.",
+                            source_pipeline_id,
+                        );
+                    },
+                    Some(source) => source,
+                };
+                // FIXME(#22512): enqueues a task; unnecessary delay.
+                window.post_message(origin, &*source, StructuredCloneData::Vector(data))
+            },
         }
     }
 
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index 788a206b63f0..b97544f92230 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -284,7 +284,18 @@ pub enum ConstellationControlMsg {
     /// PipelineId is for the parent, BrowsingContextId is for the nested browsing context
     Navigate(PipelineId, BrowsingContextId, LoadData, bool),
     /// Post a message to a given window.
-    PostMessage(PipelineId, Option<ImmutableOrigin>, Vec<u8>),
+    PostMessage {
+        /// The target of the message.
+        target: PipelineId,
+        /// The source of the message.
+        source: PipelineId,
+        /// The top level browsing context associated with the source pipeline.
+        source_browsing_context: TopLevelBrowsingContextId,
+        /// The expected origin of the target.
+        target_origin: Option<ImmutableOrigin>,
+        /// The data to be posted.
+        data: Vec<u8>,
+    },
     /// Updates the current pipeline ID of a given iframe.
     /// First PipelineId is for the parent, second is the new PipelineId for the frame.
     UpdatePipelineId(
@@ -358,7 +369,7 @@ impl fmt::Debug for ConstellationControlMsg {
             ChangeFrameVisibilityStatus(..) => "ChangeFrameVisibilityStatus",
             NotifyVisibilityChange(..) => "NotifyVisibilityChange",
             Navigate(..) => "Navigate",
-            PostMessage(..) => "PostMessage",
+            PostMessage { .. } => "PostMessage",
             UpdatePipelineId(..) => "UpdatePipelineId",
             UpdateHistoryState(..) => "UpdateHistoryState",
             RemoveHistoryStates(..) => "RemoveHistoryStates",
diff --git a/components/script_traits/script_msg.rs b/components/script_traits/script_msg.rs
index 54b52fb33d01..a63462fd83f0 100644
--- a/components/script_traits/script_msg.rs
+++ b/components/script_traits/script_msg.rs
@@ -134,7 +134,16 @@ pub enum ScriptMsg {
     /// Abort loading after sending a LoadUrl message.
     AbortLoadUrl,
     /// Post a message to the currently active window of a given browsing context.
-    PostMessage(BrowsingContextId, Option<ImmutableOrigin>, Vec<u8>),
+    PostMessage {
+        /// The target of the posted message.
+        target: BrowsingContextId,
+        /// The source of the posted message.
+        source: PipelineId,
+        /// The expected origin of the target.
+        target_origin: Option<ImmutableOrigin>,
+        /// The data to be posted.
+        data: Vec<u8>,
+    },
     /// Inform the constellation that a fragment was navigated to and whether or not it was a replacement navigation.
     NavigatedToFragment(ServoUrl, bool),
     /// HTMLIFrameElement Forward or Back traversal.
@@ -209,7 +218,7 @@ impl fmt::Debug for ScriptMsg {
             LoadComplete => "LoadComplete",
             LoadUrl(..) => "LoadUrl",
             AbortLoadUrl => "AbortLoadUrl",
-            PostMessage(..) => "PostMessage",
+            PostMessage { .. } => "PostMessage",
             NavigatedToFragment(..) => "NavigatedToFragment",
             TraverseHistory(..) => "TraverseHistory",
             PushHistoryState(..) => "PushHistoryState",