diff --git a/servo/components/style/gecko/generated/bindings.rs b/servo/components/style/gecko/generated/bindings.rs
index 5a7ec0bb7fa8..a5f1cd69fda0 100644
--- a/servo/components/style/gecko/generated/bindings.rs
+++ b/servo/components/style/gecko/generated/bindings.rs
@@ -1000,6 +1000,10 @@ extern "C" {
 extern "C" {
     pub fn Gecko_ImageValue_Create(aURI: ServoBundledURI) -> *mut ImageValue;
 }
+extern "C" {
+    pub fn Gecko_ImageValue_SizeOfIncludingThis(aImageValue: *mut ImageValue)
+     -> usize;
+}
 extern "C" {
     pub fn Gecko_SetLayerImageImageValue(image: *mut nsStyleImage,
                                          aImageValue: *mut ImageValue);
diff --git a/servo/components/style/gecko/url.rs b/servo/components/style/gecko/url.rs
index 04d3459aeec7..224a45c084a4 100644
--- a/servo/components/style/gecko/url.rs
+++ b/servo/components/style/gecko/url.rs
@@ -9,28 +9,26 @@ use gecko_bindings::structs::mozilla::css::URLValueData;
 use gecko_bindings::structs::root::mozilla::css::ImageValue;
 use gecko_bindings::structs::root::nsStyleImageRequest;
 use gecko_bindings::sugar::refptr::RefPtr;
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use parser::ParserContext;
 use servo_arc::Arc;
 use std::fmt;
 use style_traits::{ToCss, ParseError};
 
 /// A specified url() value for gecko. Gecko does not eagerly resolve SpecifiedUrls.
-#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
+#[derive(Clone, Debug, PartialEq)]
 pub struct SpecifiedUrl {
     /// The URL in unresolved string form.
     ///
     /// Refcounted since cloning this should be cheap and data: uris can be
     /// really large.
-    #[ignore_malloc_size_of = "XXX: do this once bug 1397971 lands"]
     serialization: Arc<String>,
 
     /// The URL extra data.
-    #[ignore_malloc_size_of = "RefPtr is tricky, and there aren't many of these in practise"]
     pub extra_data: RefPtr<URLExtraData>,
 
     /// Cache ImageValue, if any, so that we can reuse it while rematching a
     /// a property with this specified url value.
-    #[ignore_malloc_size_of = "XXX: do this once bug 1397971 lands"]
     pub image_value: Option<RefPtr<ImageValue>>,
 }
 trivial_to_computed_value!(SpecifiedUrl);
@@ -133,3 +131,25 @@ impl ToCss for SpecifiedUrl {
         dest.write_str(")")
     }
 }
+
+impl MallocSizeOf for SpecifiedUrl {
+    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
+        use gecko_bindings::bindings::Gecko_ImageValue_SizeOfIncludingThis;
+
+        let mut n = 0;
+
+        // XXX: measure `serialization` once bug 1397971 lands
+
+        // We ignore `extra_data`, because RefPtr is tricky, and there aren't
+        // many of them in practise (sharing is common).
+
+        if let Some(ref image_value) = self.image_value {
+            // Although this is a RefPtr, this is the primary reference because
+            // SpecifiedUrl is responsible for creating the image_value. So we
+            // measure unconditionally here.
+            n += unsafe { Gecko_ImageValue_SizeOfIncludingThis(image_value.clone().get()) };
+        }
+
+        n
+    }
+}