diff --git a/components/script/dom/bindings/proxyhandler.rs b/components/script/dom/bindings/proxyhandler.rs
index e400c1678d8..a5be8f1dc96 100644
--- a/components/script/dom/bindings/proxyhandler.rs
+++ b/components/script/dom/bindings/proxyhandler.rs
@@ -17,7 +17,7 @@ use js::jsapi::JS_GetPropertyDescriptorById;
 use js::jsapi::MutableHandleObject;
 use js::jsapi::{Handle, HandleId, HandleObject, MutableHandle, ObjectOpResult};
 use js::jsapi::{JSContext, JSObject, JSPROP_GETTER, PropertyDescriptor, DOMProxyShadowsResult};
-use js::jsapi::{JSErrNum, JS_StrictPropertyStub};
+use js::jsapi::{JSErrNum, JS_StrictPropertyStub, JS_AlreadyHasOwnPropertyById};
 use js::jsapi::{JS_DefinePropertyById, JS_NewObjectWithGivenProto, SetDOMProxyInformation};
 use js::jsval::ObjectValue;
 use libc;
@@ -26,12 +26,26 @@ use std::{mem, ptr};
 static JSPROXYSLOT_EXPANDO: u32 = 0;
 
 /// Determine if this id shadows any existing properties for this proxy.
-pub unsafe extern "C" fn shadow_check_callback(_cx: *mut JSContext,
-                                               _object: HandleObject,
-                                               _id: HandleId)
+pub unsafe extern "C" fn shadow_check_callback(cx: *mut JSContext,
+                                               object: HandleObject,
+                                               id: HandleId)
                                                -> DOMProxyShadowsResult {
-    // XXX implement me
-    DOMProxyShadowsResult::ShadowCheckFailed
+    // TODO: support OverrideBuiltins when #12978 is fixed.
+
+    rooted!(in(cx) let expando = get_expando_object(object));
+    if !expando.get().is_null() {
+        let mut has_own = false;
+        if !JS_AlreadyHasOwnPropertyById(cx, expando.handle(), id, &mut has_own) {
+            return DOMProxyShadowsResult::ShadowCheckFailed;
+        }
+
+        if has_own {
+            return DOMProxyShadowsResult::ShadowsViaDirectExpando;
+        }
+    }
+
+    // Our expando, if any, didn't shadow, so we're not shadowing at all.
+    DOMProxyShadowsResult::DoesntShadow
 }
 
 /// Initialize the infrastructure for DOM proxy objects.
diff --git a/tests/html/bindings_perf.html b/tests/html/bindings_perf.html
index d50cf940c32..123c53d6c00 100644
--- a/tests/html/bindings_perf.html
+++ b/tests/html/bindings_perf.html
@@ -1,6 +1,7 @@
 <button onclick="void_method()">measure void method</button>
 <button onclick="int_getter()">measure int getter</button>
 <button onclick="firstChild_getter()">measure firstChild getter</button>
+<button onclick="proxy_firstChild_getter()">measure proxy firstChild getter</button>
 <script>
 var t = 'TestBinding' in window ? (new TestBinding()) : (new TextEncoder());
 function void_method() {
@@ -33,4 +34,15 @@ function firstChild_getter() {
   var stop = new Date();
   console.log('firstChild getter: ' + ((stop - start) / count * 1e6) + 'ns');
 }
+
+function proxy_firstChild_getter() {
+  var n = document;
+  var start = new Date();
+  var count = 1000000;
+  for (var i = 0; i < count; i++) {
+    var a = n.firstChild;
+  }
+  var stop = new Date();
+  console.log('proxy firstChild getter: ' + ((stop - start) / count * 1e6) + 'ns');
+}
 </script>