diff --git a/components/script/dom/browsingcontext.rs b/components/script/dom/browsingcontext.rs
index 9f0be02e4842e..5e523d6982a4c 100644
--- a/components/script/dom/browsingcontext.rs
+++ b/components/script/dom/browsingcontext.rs
@@ -3,13 +3,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use dom::bindings::conversions::{ToJSValConvertible, root_from_handleobject};
+use dom::bindings::error::{Error, throw_dom_exception};
+use dom::bindings::inheritance::Castable;
 use dom::bindings::js::{JS, Root, RootedReference};
 use dom::bindings::proxyhandler::{fill_property_descriptor, get_property_descriptor};
 use dom::bindings::reflector::{DomObject, Reflector};
 use dom::bindings::trace::JSTraceable;
 use dom::bindings::utils::WindowProxyHandler;
 use dom::bindings::utils::get_array_index_from_id;
+use dom::dissimilaroriginwindow::DissimilarOriginWindow;
 use dom::element::Element;
+use dom::globalscope::GlobalScope;
 use dom::window::Window;
 use js::JSCLASS_IS_GLOBAL;
 use js::glue::{CreateWrapperProxyHandler, ProxyTraps, NewWindowProxy};
@@ -18,12 +22,13 @@ use js::jsapi::{Handle, HandleId, HandleObject, HandleValue};
 use js::jsapi::{JSAutoCompartment, JSContext, JSErrNum, JSFreeOp, JSObject};
 use js::jsapi::{JSPROP_READONLY, JSTracer, JS_DefinePropertyById};
 use js::jsapi::{JS_ForwardGetPropertyTo, JS_ForwardSetPropertyTo};
-use js::jsapi::{JS_GetOwnPropertyDescriptorById, JS_HasPropertyById};
-use js::jsapi::{JS_TransplantObject, SetWindowProxy};
+use js::jsapi::{JS_GetOwnPropertyDescriptorById, JS_HasPropertyById, JS_HasOwnPropertyById};
+use js::jsapi::{JS_IsExceptionPending, JS_TransplantObject, SetWindowProxy};
 use js::jsapi::{MutableHandle, MutableHandleObject, MutableHandleValue};
 use js::jsapi::{ObjectOpResult, PropertyDescriptor};
 use js::jsval::{UndefinedValue, PrivateValue};
 use js::rust::get_object_class;
+use msg::constellation_msg::PipelineId;
 use std::cell::Cell;
 use std::ptr;
 
@@ -39,6 +44,13 @@ pub struct BrowsingContext {
     /// changes Window.
     reflector: Reflector,
 
+    /// The pipeline id of the currently active document.
+    /// May be None, when the currently active document is in another script thread.
+    /// We do not try to keep the pipeline id for documents in other threads,
+    /// as this would require the constellation notifying many script threads about
+    /// the change, which could be expensive.
+    currently_active: Cell<Option<PipelineId>>,
+
     /// Has this browsing context been discarded?
     discarded: Cell<bool>,
 
@@ -47,9 +59,10 @@ pub struct BrowsingContext {
 }
 
 impl BrowsingContext {
-    pub fn new_inherited(frame_element: Option<&Element>) -> BrowsingContext {
+    pub fn new_inherited(currently_active: PipelineId, frame_element: Option<&Element>) -> BrowsingContext {
         BrowsingContext {
             reflector: Reflector::new(),
+            currently_active: Cell::new(Some(currently_active)),
             discarded: Cell::new(false),
             frame_element: frame_element.map(JS::from_ref),
         }
@@ -72,7 +85,8 @@ impl BrowsingContext {
             assert!(!window_proxy.is_null());
 
             // Create a new browsing context.
-            let mut browsing_context = box BrowsingContext::new_inherited(frame_element);
+            let currently_active = window.global().pipeline_id();
+            let mut browsing_context = box BrowsingContext::new_inherited(currently_active, frame_element);
 
             // The window proxy owns the browsing context.
             // When we finalize the window proxy, it drops the browsing context it owns.
@@ -104,10 +118,10 @@ impl BrowsingContext {
     /// Change the Window that this browsing context's WindowProxy resolves to.
     // TODO: support setting the window proxy to a dummy value,
     // to handle the case when the active document is in another script thread.
-    pub fn set_window_proxy(&self, window: &Window) {
+    fn set_window_proxy(&self, window: &GlobalScope, traps: &ProxyTraps) {
         unsafe {
             debug!("Setting window proxy of {:p}.", self);
-            let WindowProxyHandler(handler) = window.windowproxy_handler();
+            let handler = CreateWrapperProxyHandler(traps);
             assert!(!handler.is_null());
 
             let cx = window.get_cx();
@@ -144,6 +158,22 @@ impl BrowsingContext {
         }
     }
 
+    pub fn set_currently_active(&self, window: &Window) {
+        let globalscope = window.upcast();
+        self.set_window_proxy(&*globalscope, &PROXY_HANDLER);
+        self.currently_active.set(Some(globalscope.pipeline_id()));
+    }
+
+    pub fn unset_currently_active(&self) {
+        let window = DissimilarOriginWindow::new(self);
+        self.set_window_proxy(&*window.upcast(), &XORIGIN_PROXY_HANDLER);
+        self.currently_active.set(None);
+    }
+
+    pub fn currently_active(&self) -> Option<PipelineId> {
+        self.currently_active.get()
+    }
+
     pub fn window_proxy(&self) -> *mut JSObject {
         let window_proxy = self.reflector.get_jsobject();
         assert!(!window_proxy.get().is_null());
@@ -332,6 +362,145 @@ static PROXY_HANDLER: ProxyTraps = ProxyTraps {
     isConstructor: None,
 };
 
+#[allow(unsafe_code)]
+pub fn new_window_proxy_handler() -> WindowProxyHandler {
+    unsafe {
+        WindowProxyHandler(CreateWrapperProxyHandler(&PROXY_HANDLER))
+    }
+}
+
+// The proxy traps for cross-origin windows.
+// These traps often throw security errors, and only pass on calls to methods
+// defined in the DissimilarOriginWindow IDL.
+
+#[allow(unsafe_code)]
+unsafe fn throw_security_error(cx: *mut JSContext) -> bool {
+    if !JS_IsExceptionPending(cx) {
+        let global = GlobalScope::from_context(cx);
+        throw_dom_exception(cx, &*global, Error::Security);
+    }
+    false
+}
+
+#[allow(unsafe_code)]
+unsafe extern "C" fn has_xorigin(cx: *mut JSContext,
+                                 proxy: HandleObject,
+                                 id: HandleId,
+                                 bp: *mut bool)
+                                 -> bool
+{
+    rooted!(in(cx) let target = GetProxyPrivate(*proxy.ptr).to_object());
+    let mut found = false;
+    JS_HasOwnPropertyById(cx, target.handle(), id, &mut found);
+    if found {
+        *bp = true;
+        true
+    } else {
+        throw_security_error(cx)
+    }
+}
+
+#[allow(unsafe_code)]
+unsafe extern "C" fn get_xorigin(cx: *mut JSContext,
+                                 proxy: HandleObject,
+                                 receiver: HandleValue,
+                                 id: HandleId,
+                                 vp: MutableHandleValue)
+                                 -> bool
+{
+    let mut found = false;
+    has_xorigin(cx, proxy, id, &mut found);
+    found && get(cx, proxy, receiver, id, vp)
+}
+
+#[allow(unsafe_code)]
+unsafe extern "C" fn set_xorigin(cx: *mut JSContext,
+                                 _: HandleObject,
+                                 _: HandleId,
+                                 _: HandleValue,
+                                 _: HandleValue,
+                                 _: *mut ObjectOpResult)
+                                 -> bool
+{
+    throw_security_error(cx)
+}
+
+#[allow(unsafe_code)]
+unsafe extern "C" fn delete_xorigin(cx: *mut JSContext,
+                                    _: HandleObject,
+                                    _: HandleId,
+                                    _: *mut ObjectOpResult)
+                                    -> bool
+{
+    throw_security_error(cx)
+}
+
+#[allow(unsafe_code)]
+unsafe extern "C" fn getOwnPropertyDescriptor_xorigin(cx: *mut JSContext,
+                                                      proxy: HandleObject,
+                                                      id: HandleId,
+                                                      desc: MutableHandle<PropertyDescriptor>)
+                                                      -> bool
+{
+    let mut found = false;
+    has_xorigin(cx, proxy, id, &mut found);
+    found && getOwnPropertyDescriptor(cx, proxy, id, desc)
+}
+
+#[allow(unsafe_code)]
+unsafe extern "C" fn defineProperty_xorigin(cx: *mut JSContext,
+                                            _: HandleObject,
+                                            _: HandleId,
+                                            _: Handle<PropertyDescriptor>,
+                                            _: *mut ObjectOpResult)
+                                            -> bool
+{
+    throw_security_error(cx)
+}
+
+#[allow(unsafe_code)]
+unsafe extern "C" fn preventExtensions_xorigin(cx: *mut JSContext,
+                                               _: HandleObject,
+                                               _: *mut ObjectOpResult)
+                                               -> bool
+{
+    throw_security_error(cx)
+}
+
+static XORIGIN_PROXY_HANDLER: ProxyTraps = ProxyTraps {
+    enter: None,
+    getOwnPropertyDescriptor: Some(getOwnPropertyDescriptor_xorigin),
+    defineProperty: Some(defineProperty_xorigin),
+    ownPropertyKeys: None,
+    delete_: Some(delete_xorigin),
+    enumerate: None,
+    getPrototypeIfOrdinary: None,
+    preventExtensions: Some(preventExtensions_xorigin),
+    isExtensible: None,
+    has: Some(has_xorigin),
+    get: Some(get_xorigin),
+    set: Some(set_xorigin),
+    call: None,
+    construct: None,
+    getPropertyDescriptor: Some(getOwnPropertyDescriptor_xorigin),
+    hasOwn: Some(has_xorigin),
+    getOwnEnumerablePropertyKeys: None,
+    nativeCall: None,
+    hasInstance: None,
+    objectClassIs: None,
+    className: None,
+    fun_toString: None,
+    boxedValue_unbox: None,
+    defaultValue: None,
+    trace: Some(trace),
+    finalize: Some(finalize),
+    objectMoved: None,
+    isCallable: None,
+    isConstructor: None,
+};
+
+// How WindowProxy objects are garbage collected.
+
 #[allow(unsafe_code)]
 unsafe extern fn finalize(_fop: *mut JSFreeOp, obj: *mut JSObject) {
     let this = GetProxyExtra(obj, 0).to_private() as *mut BrowsingContext;
@@ -354,9 +523,3 @@ unsafe extern fn trace(trc: *mut JSTracer, obj: *mut JSObject) {
     (*this).trace(trc);
 }
 
-#[allow(unsafe_code)]
-pub fn new_window_proxy_handler() -> WindowProxyHandler {
-    unsafe {
-        WindowProxyHandler(CreateWrapperProxyHandler(&PROXY_HANDLER))
-    }
-}
diff --git a/components/script/dom/dissimilaroriginlocation.rs b/components/script/dom/dissimilaroriginlocation.rs
new file mode 100644
index 0000000000000..6f5eda8c40250
--- /dev/null
+++ b/components/script/dom/dissimilaroriginlocation.rs
@@ -0,0 +1,80 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use dom::bindings::codegen::Bindings::DissimilarOriginLocationBinding;
+use dom::bindings::codegen::Bindings::DissimilarOriginLocationBinding::DissimilarOriginLocationMethods;
+use dom::bindings::error::{Error, ErrorResult, Fallible};
+use dom::bindings::js::{JS, Root};
+use dom::bindings::reflector::Reflector;
+use dom::bindings::reflector::reflect_dom_object;
+use dom::bindings::str::DOMString;
+use dom::bindings::str::USVString;
+use dom::dissimilaroriginwindow::DissimilarOriginWindow;
+
+/// Represents a dissimilar-origin `Location` that exists in another script thread.
+///
+/// Since the `Location` is in a different script thread, we cannot access it
+/// directly, but some of its accessors (for example setting `location.href`)
+/// still need to function.
+
+#[dom_struct]
+pub struct DissimilarOriginLocation {
+    /// The reflector. Once we have XOWs, this will have a cross-origin
+    /// wrapper placed around it.
+    reflector: Reflector,
+
+    /// The window associated with this location.
+    window: JS<DissimilarOriginWindow>,
+}
+
+impl DissimilarOriginLocation {
+    #[allow(unrooted_must_root)]
+    fn new_inherited(window: &DissimilarOriginWindow) -> DissimilarOriginLocation {
+        DissimilarOriginLocation {
+            reflector: Reflector::new(),
+            window: JS::from_ref(window),
+        }
+    }
+
+    pub fn new(window: &DissimilarOriginWindow) -> Root<DissimilarOriginLocation> {
+        reflect_dom_object(box DissimilarOriginLocation::new_inherited(window),
+                           window,
+                           DissimilarOriginLocationBinding::Wrap)
+    }
+}
+
+impl DissimilarOriginLocationMethods for DissimilarOriginLocation {
+    // https://html.spec.whatwg.org/multipage/#dom-location-href
+    fn GetHref(&self) -> Fallible<USVString> {
+        Err(Error::Security)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-location-href
+    fn SetHref(&self, _: USVString) -> ErrorResult {
+        // TODO: setting href on a cross-origin window should succeed?
+        Err(Error::Security)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-location-assign
+    fn Assign(&self, _: USVString) -> Fallible<()> {
+        // TODO: setting href on a cross-origin window should succeed?
+        Err(Error::Security)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-location-replace
+    fn Replace(&self, _: USVString) -> Fallible<()> {
+        // TODO: replacing href on a cross-origin window should succeed?
+        Err(Error::Security)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-location-reload
+    fn Reload(&self) -> Fallible<()> {
+        Err(Error::Security)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-location-href
+    fn Stringifier(&self) -> Fallible<DOMString> {
+        Err(Error::Security)
+    }
+}
diff --git a/components/script/dom/dissimilaroriginwindow.rs b/components/script/dom/dissimilaroriginwindow.rs
new file mode 100644
index 0000000000000..518adb0f34167
--- /dev/null
+++ b/components/script/dom/dissimilaroriginwindow.rs
@@ -0,0 +1,140 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use dom::bindings::codegen::Bindings::DissimilarOriginWindowBinding;
+use dom::bindings::codegen::Bindings::DissimilarOriginWindowBinding::DissimilarOriginWindowMethods;
+use dom::bindings::js::{JS, MutNullableJS, Root};
+use dom::bindings::reflector::DomObject;
+use dom::bindings::str::DOMString;
+use dom::browsingcontext::BrowsingContext;
+use dom::dissimilaroriginlocation::DissimilarOriginLocation;
+use dom::globalscope::GlobalScope;
+use ipc_channel::ipc;
+use js::jsapi::{JSContext, HandleValue};
+use js::jsval::{JSVal, UndefinedValue};
+use msg::constellation_msg::PipelineId;
+
+/// Represents a dissimilar-origin `Window` that exists in another script thread.
+///
+/// Since the `Window` is in a different script thread, we cannot access it
+/// directly, but some of its accessors (for example `window.parent`)
+/// still need to function.
+///
+/// In `browsingcontext.rs`, we create a custom window proxy for these windows,
+/// that throws security exceptions for most accessors. This is not a replacement
+/// for XOWs, but provides belt-and-braces security.
+#[dom_struct]
+pub struct DissimilarOriginWindow {
+    /// The global for this window.
+    globalscope: GlobalScope,
+
+    /// The browsing context this window is part of.
+    browsing_context: JS<BrowsingContext>,
+
+    /// The location of this window, initialized lazily.
+    location: MutNullableJS<DissimilarOriginLocation>,
+}
+
+impl DissimilarOriginWindow {
+    #[allow(unsafe_code)]
+    pub fn new(browsing_context: &BrowsingContext) -> Root<DissimilarOriginWindow> {
+        let globalscope = browsing_context.global();
+        let cx = globalscope.get_cx();
+        // Any timer events fired on this window are ignored.
+        let (timer_event_chan, _) = ipc::channel().unwrap();
+        let win = box DissimilarOriginWindow {
+            globalscope: GlobalScope::new_inherited(PipelineId::new(),
+                                                    globalscope.devtools_chan().cloned(),
+                                                    globalscope.mem_profiler_chan().clone(),
+                                                    globalscope.time_profiler_chan().clone(),
+                                                    globalscope.constellation_chan().clone(),
+                                                    globalscope.scheduler_chan().clone(),
+                                                    globalscope.resource_threads().clone(),
+                                                    timer_event_chan),
+            browsing_context: JS::from_ref(browsing_context),
+            location: MutNullableJS::new(None),
+        };
+        unsafe { DissimilarOriginWindowBinding::Wrap(cx, win) }
+    }
+}
+
+impl DissimilarOriginWindowMethods for DissimilarOriginWindow {
+    // https://html.spec.whatwg.org/multipage/#dom-window
+    fn Window(&self) -> Root<BrowsingContext> {
+        Root::from_ref(&*self.browsing_context)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-self
+    fn Self_(&self) -> Root<BrowsingContext> {
+        Root::from_ref(&*self.browsing_context)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-frames
+    fn Frames(&self) -> Root<BrowsingContext> {
+        Root::from_ref(&*self.browsing_context)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-parent
+    fn GetParent(&self) -> Option<Root<BrowsingContext>> {
+        // TODO: implement window.parent correctly for x-origin windows.
+        Some(Root::from_ref(&*self.browsing_context))
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-top
+    fn GetTop(&self) -> Option<Root<BrowsingContext>> {
+        // TODO: implement window.top correctly for x-origin windows.
+        Some(Root::from_ref(&*self.browsing_context))
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-length
+    fn Length(&self) -> u32 {
+        // TODO: Implement x-origin length
+        0
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-window-close
+    fn Close(&self) {
+        // TODO: Implement x-origin close
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-window-closed
+    fn Closed(&self) -> bool {
+        // TODO: Implement x-origin close
+        false
+    }
+
+    #[allow(unsafe_code)]
+    // https://html.spec.whatwg.org/multipage/#dom-window-postmessage
+    unsafe fn PostMessage(&self, _: *mut JSContext, _: HandleValue, _: DOMString) {
+        // TODO: Implement x-origin postMessage
+    }
+
+    #[allow(unsafe_code)]
+    // https://html.spec.whatwg.org/multipage/#dom-opener
+    unsafe fn Opener(&self, _: *mut JSContext) -> JSVal {
+        // TODO: Implement x-origin opener
+        UndefinedValue()
+    }
+
+    #[allow(unsafe_code)]
+    // https://html.spec.whatwg.org/multipage/#dom-opener
+    unsafe fn SetOpener(&self, _: *mut JSContext, _: HandleValue) {
+        // TODO: Implement x-origin opener
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-window-blur
+    fn Blur(&self) {
+        // TODO: Implement x-origin blur
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-focus
+    fn Focus(&self) {
+        // TODO: Implement x-origin focus
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-location
+    fn Location(&self) -> Root<DissimilarOriginLocation> {
+        self.location.or_init(|| DissimilarOriginLocation::new(self))
+    }
+}
diff --git a/components/script/dom/mod.rs b/components/script/dom/mod.rs
index 4b3c0c02cbbf3..7b96ea2523d88 100644
--- a/components/script/dom/mod.rs
+++ b/components/script/dom/mod.rs
@@ -258,6 +258,8 @@ pub mod csssupportsrule;
 pub mod cssviewportrule;
 pub mod customevent;
 pub mod dedicatedworkerglobalscope;
+pub mod dissimilaroriginlocation;
+pub mod dissimilaroriginwindow;
 pub mod document;
 pub mod documentfragment;
 pub mod documenttype;
diff --git a/components/script/dom/webidls/DissimilarOriginLocation.webidl b/components/script/dom/webidls/DissimilarOriginLocation.webidl
new file mode 100644
index 0000000000000..8eca6a790acc2
--- /dev/null
+++ b/components/script/dom/webidls/DissimilarOriginLocation.webidl
@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+
+// This is a Servo-specific interface, used to represent locations
+// that are not similar-origin, so live in another script thread.
+// It is based on the interface for Window, but only contains the
+// accessors that do not throw security exceptions when called
+// cross-origin.
+//
+// Note that similar-origin locations are kept in the same script
+// thread, so this mechanism cannot be relied upon as the only
+// way to enforce security policy.
+
+// https://html.spec.whatwg.org/multipage/#location
+[Unforgeable, NoInterfaceObject] interface DissimilarOriginLocation {
+  [Throws] attribute USVString href;
+  [Throws] void assign(USVString url);
+  [Throws] void replace(USVString url);
+  [Throws] void reload();
+  [Throws] stringifier;
+
+  // TODO: finish this interface
+};
diff --git a/components/script/dom/webidls/DissimilarOriginWindow.webidl b/components/script/dom/webidls/DissimilarOriginWindow.webidl
new file mode 100644
index 0000000000000..6aeb5c7d1b220
--- /dev/null
+++ b/components/script/dom/webidls/DissimilarOriginWindow.webidl
@@ -0,0 +1,32 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This is a Servo-specific interface, used to represent windows
+// that are not similar-origin, so live in another script thread.
+// It is based on the interface for Window, but only contains the
+// accessors that do not throw security exceptions when called
+// cross-origin.
+//
+// Note that similar-origin windows are kept in the same script
+// thread, so this mechanism cannot be relied upon as the only
+// way to enforce security policy.
+
+// https://html.spec.whatwg.org/multipage/#window
+[Global, NoInterfaceObject]
+interface DissimilarOriginWindow : GlobalScope {
+  [Unforgeable] readonly attribute WindowProxy window;
+  [BinaryName="Self_", Replaceable] readonly attribute WindowProxy self;
+  [Unforgeable] readonly attribute WindowProxy? parent;
+  [Unforgeable] readonly attribute WindowProxy? top;
+  [Replaceable] readonly attribute WindowProxy frames;
+  [Replaceable] readonly attribute unsigned long length;
+  [Unforgeable] readonly attribute DissimilarOriginLocation location;
+
+  void close();
+  readonly attribute boolean closed;
+  void postMessage(any message, DOMString targetOrigin);
+  attribute any opener;
+  void blur();
+  void focus();
+};
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index d1eba0245e0c6..1fbe57ba16510 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -1514,7 +1514,10 @@ impl Window {
         // Suspend timer events.
         self.upcast::<GlobalScope>().suspend();
 
-        // TODO: set the window proxy to resolve to an object which throws security errors. #15233
+        // Set the window proxy to be a cross-origin window.
+        if self.browsing_context().currently_active() == Some(self.global().pipeline_id()) {
+            self.browsing_context().unset_currently_active();
+        }
 
         // A hint to the JS runtime that now would be a good time to
         // GC any unreachable objects generated by user script,
@@ -1528,7 +1531,7 @@ impl Window {
         self.upcast::<GlobalScope>().resume();
 
         // Set the window proxy to be this object.
-        self.browsing_context().set_window_proxy(&self);
+        self.browsing_context().set_currently_active(self);
 
         // Push the document title to the compositor since we are
         // activating this document due to a navigation.
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index fa83eaed63191..ac093a7a5df9e 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1678,7 +1678,7 @@ impl ScriptThread {
             },
             hash_map::Entry::Occupied(entry) => {
                 let browsing_context = entry.get();
-                browsing_context.set_window_proxy(&window);
+                browsing_context.set_currently_active(&*window);
                 window.init_browsing_context(browsing_context);
             },
         }
@@ -1782,7 +1782,9 @@ impl ScriptThread {
             ServoParser::parse_html_document(&document, parse_input, final_url);
         }
 
-        if incomplete.activity != DocumentActivity::FullyActive {
+        if incomplete.activity == DocumentActivity::FullyActive {
+            window.resume();
+        } else {
             window.suspend();
         }
 
diff --git a/tests/wpt/metadata/html/browsers/origin/cross-origin-objects/cross-origin-objects-exceptions.html.ini b/tests/wpt/metadata/html/browsers/origin/cross-origin-objects/cross-origin-objects-exceptions.html.ini
new file mode 100644
index 0000000000000..487bf415faf11
--- /dev/null
+++ b/tests/wpt/metadata/html/browsers/origin/cross-origin-objects/cross-origin-objects-exceptions.html.ini
@@ -0,0 +1,47 @@
+[cross-origin-objects-exceptions.html]
+  type: testharness
+  [Basic sanity-checking]
+    expected: FAIL
+
+  [Only whitelisted properties are accessible cross-origin]
+    expected: PASS
+
+  [[[GetPrototypeOf\]\] should return null]
+    expected: FAIL
+
+  [[[SetPrototypeOf\]\] should throw]
+    expected: FAIL
+
+  [[[PreventExtensions\]\] should throw for cross-origin objects]
+    expected: FAIL
+
+  [[[GetOwnProperty\]\] - Property descriptors for cross-origin properties should be set up correctly]
+    expected: FAIL
+
+  [[[Delete\]\] Should throw on cross-origin objects]
+    expected: FAIL
+
+  [[[DefineOwnProperty\]\] Should throw for cross-origin objects]
+    expected: FAIL
+
+  [[[Enumerate\]\] should return an empty iterator]
+    expected: FAIL
+
+  [[[OwnPropertyKeys\]\] should return all properties from cross-origin objects]
+    expected: FAIL
+
+  [Cross-origin functions get local Function.prototype]
+    expected: FAIL
+
+  [Cross-origin Window accessors get local Function.prototype]
+    expected: FAIL
+
+  [Same-origin observers get different functions for cross-origin objects]
+    expected: FAIL
+
+  [Same-origin observers get different accessors for cross-origin Window]
+    expected: FAIL
+
+  [Same-origin observers get different accessors for cross-origin Location]
+    expected: FAIL
+
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index c73fdd4a39073..ad2e7aa9492fb 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -12721,6 +12721,14 @@
      {}
     ]
    ],
+   "mozilla/cross-origin-objects/cross-origin-objects.html": [
+    [
+     "/_mozilla/mozilla/cross-origin-objects/cross-origin-objects.html",
+     {
+      "timeout": "long"
+     }
+    ]
+   ],
    "mozilla/deterministic-raf.html": [
     [
      "/_mozilla/mozilla/deterministic-raf.html",
@@ -25350,6 +25358,10 @@
    "f1029d519aa7017a1a3d18a891a0774b9a39f847",
    "testharness"
   ],
+  "mozilla/cross-origin-objects/cross-origin-objects.html": [
+   "7a73c49fafc945245f6a13a51dc2c5f1096bc15c",
+   "testharness"
+  ],
   "mozilla/details_ui_closed.html": [
    "2acbe3afbec267bad4dd986803e636740a707507",
    "reftest"
diff --git a/tests/wpt/mozilla/meta/mozilla/cross-origin-objects/cross-origin-objects.html.ini b/tests/wpt/mozilla/meta/mozilla/cross-origin-objects/cross-origin-objects.html.ini
new file mode 100644
index 0000000000000..6c0df60a409b8
--- /dev/null
+++ b/tests/wpt/mozilla/meta/mozilla/cross-origin-objects/cross-origin-objects.html.ini
@@ -0,0 +1,5 @@
+[cross-origin-objects.html]
+  type: testharness
+
+  [Parentage of cross-origin windows]
+    expected: FAIL
diff --git a/tests/wpt/mozilla/tests/mozilla/cross-origin-objects/cross-origin-objects.html b/tests/wpt/mozilla/tests/mozilla/cross-origin-objects/cross-origin-objects.html
new file mode 100644
index 0000000000000..eec9232845d12
--- /dev/null
+++ b/tests/wpt/mozilla/tests/mozilla/cross-origin-objects/cross-origin-objects.html
@@ -0,0 +1,93 @@
+<!doctype html>
+<meta charset=utf-8>
+<meta name="timeout" content="long">
+<title>Cross-origin behavior of Window and Location</title>
+<link rel="author" title="Bobby Holley (:bholley)" href="bobbyholley@gmail.com">
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#security-window">
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#security-location">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id=log></div>
+<iframe id="B"></iframe>
+<iframe id="C"></iframe>
+<script>
+
+/*
+ * This is a stripped down version of
+ * /html/browsers/origin/cross-origin-objects/cross-origin-objects.html,
+ * which runs the tests in a dissimilar-origin iframe.
+ * This is a temporary work-around until the web-platform-tests supports
+ * more than one domain.
+ */
+
+var B = document.getElementById('B').contentWindow;
+var C = document.getElementById('C').contentWindow;
+
+document.getElementById('B').src = "/common/blank.html";
+document.getElementById('C').src = "http://127.0.0.1:8000/common/blank.html";
+
+/*
+ * Parentage
+ */
+
+async_test(function(t) { C.addEventListener("load", t.step_func(function() {
+  assert_equals(B.parent, window, "window.parent works same-origin");
+  assert_equals(C.parent, window, "window.parent works cross-origin");
+  assert_equals(B.top, window, "window.top works same-origin");
+  assert_equals(C.top, window, "window.top works cross-origin");
+  t.done();
+}))}, "Parentage of cross-origin windows");
+
+/*
+ * Whitelist behavior.
+ *
+ * Also tests for [[GetOwnProperty]] and [[HasOwnProperty]] behavior.
+ */
+
+var whitelistedWindowIndices = ['0', '1'];
+var whitelistedWindowPropNames = ['location', 'postMessage', 'window', 'frames', 'self', 'top', 'parent',
+                                  'opener', 'closed', 'close', 'blur', 'focus', 'length'];
+whitelistedWindowPropNames = whitelistedWindowPropNames.concat(whitelistedWindowIndices);
+whitelistedWindowPropNames.sort();
+var whitelistedLocationPropNames = ['href', 'replace'];
+whitelistedLocationPropNames.sort();
+var whitelistedSymbols = [Symbol.toStringTag, Symbol.hasInstance,
+                          Symbol.isConcatSpreadable];
+var whitelistedWindowProps = whitelistedWindowPropNames.concat(whitelistedSymbols);
+
+async_test(function(t) { C.addEventListener("load", t.step_func(function() {
+  for (var prop in window) {
+    if (whitelistedWindowProps.indexOf(prop) != -1) {
+      C[prop]; // Shouldn't throw.
+      Object.getOwnPropertyDescriptor(C, prop); // Shouldn't throw.
+      assert_true(Object.prototype.hasOwnProperty.call(C, prop), "hasOwnProperty for " + String(prop));
+    } else {
+      assert_throws("SecurityError", function() { C[prop]; }, "Should throw when accessing " + String(prop) + " on Window");
+      assert_throws("SecurityError", function() { Object.getOwnPropertyDescriptor(C, prop); },
+                    "Should throw when accessing property descriptor for " + prop + " on Window");
+      assert_throws("SecurityError", function() { Object.prototype.hasOwnProperty.call(C, prop); },
+                    "Should throw when invoking hasOwnProperty for " + prop + " on Window");
+    }
+    if (prop != 'location')
+      assert_throws("SecurityError", function() { C[prop] = undefined; }, "Should throw when writing to " + prop + " on Window");
+  }
+  for (var prop in location) {
+    if (prop == 'replace') {
+      C.location[prop]; // Shouldn't throw.
+      Object.getOwnPropertyDescriptor(C.location, prop); // Shouldn't throw.
+      assert_true(Object.prototype.hasOwnProperty.call(C.location, prop), "hasOwnProperty for " + prop);
+    }
+    else {
+      assert_throws("SecurityError", function() { C[prop]; }, "Should throw when accessing " + prop + " on Location");
+      assert_throws("SecurityError", function() { Object.getOwnPropertyDescriptor(C, prop); },
+                    "Should throw when accessing property descriptor for " + prop + " on Location");
+      assert_throws("SecurityError", function() { Object.prototype.hasOwnProperty.call(C, prop); },
+                    "Should throw when invoking hasOwnProperty for " + prop + " on Location");
+    }
+    if (prop != 'href')
+      assert_throws("SecurityError", function() { C[prop] = undefined; }, "Should throw when writing to " + prop + " on Location");
+  }
+  t.done();
+}))}, "Only whitelisted properties are accessible cross-origin");
+
+</script>