--- a/servo/components/style/stylist.rs
+++ b/servo/components/style/stylist.rs
@@ -1065,44 +1065,42 @@ impl Stylist {
     /// Returns the sheet origins that were actually affected.
     ///
     /// This means that we may need to rebuild style data even if the
     /// stylesheets haven't changed.
     ///
     /// Also, the device that arrives here may need to take the viewport rules
     /// into account.
     ///
-    /// feature = "servo" because gecko only has one device, and manually tracks
-    /// when the device is dirty.
-    ///
-    /// FIXME(emilio): The semantics of the device for Servo and Gecko are
-    /// different enough we may want to unify them.
-    #[cfg(feature = "servo")]
+    /// For Gecko, this is called when XBL bindings are used by different
+    /// documents.
     pub fn set_device(
         &mut self,
         mut device: Device,
         guard: &SharedRwLockReadGuard,
     ) -> OriginSet {
-        let cascaded_rule = {
-            let stylesheets = self.stylesheets.iter();
+        if viewport_rule::enabled() {
+            let cascaded_rule = {
+                let stylesheets = self.stylesheets.iter();
 
-            ViewportRule {
-                declarations: viewport_rule::Cascade::from_stylesheets(
-                    stylesheets.clone(),
-                    guard,
-                    &device
-                ).finish(),
+                ViewportRule {
+                    declarations: viewport_rule::Cascade::from_stylesheets(
+                        stylesheets.clone(),
+                        guard,
+                        &device
+                    ).finish(),
+                }
+            };
+
+            self.viewport_constraints =
+                ViewportConstraints::maybe_new(&device, &cascaded_rule, self.quirks_mode);
+
+            if let Some(ref constraints) = self.viewport_constraints {
+                device.account_for_viewport_rule(constraints);
             }
-        };
-
-        self.viewport_constraints =
-            ViewportConstraints::maybe_new(&device, &cascaded_rule, self.quirks_mode);
-
-        if let Some(ref constraints) = self.viewport_constraints {
-            device.account_for_viewport_rule(constraints);
         }
 
         self.device = device;
         self.media_features_change_changed_style(guard)
     }
 
     /// Returns whether, given a media feature change, any previously-applicable
     /// style has become non-applicable, or vice-versa for each origin.

--- a/servo/ports/geckolib/glue.rs
+++ b/servo/ports/geckolib/glue.rs
@@ -97,17 +97,17 @@ use style::gecko_bindings::structs::nsID
 use style::gecko_bindings::structs::nsStyleTransformMatrix::MatrixTransformOperator;
 use style::gecko_bindings::structs::nsTArray;
 use style::gecko_bindings::structs::nsresult;
 use style::gecko_bindings::sugar::ownership::{FFIArcHelpers, HasFFI, HasArcFFI};
 use style::gecko_bindings::sugar::ownership::{HasSimpleFFI, Strong};
 use style::gecko_bindings::sugar::refptr::RefPtr;
 use style::gecko_properties::style_structs;
 use style::invalidation::element::restyle_hints;
-use style::media_queries::{MediaList, parse_media_query_list};
+use style::media_queries::{Device, MediaList, parse_media_query_list};
 use style::parser::{ParserContext, self};
 use style::properties::{CascadeFlags, ComputedValues, Importance};
 use style::properties::{IS_FIELDSET_CONTENT, IS_LINK, IS_VISITED_LINK, LonghandIdSet};
 use style::properties::{PropertyDeclaration, PropertyDeclarationBlock, PropertyId, ShorthandId};
 use style::properties::{SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP, SourcePropertyDeclaration, StyleBuilder};
 use style::properties::PROHIBIT_DISPLAY_CONTENTS;
 use style::properties::animated_properties::{AnimatableLonghand, AnimationValue};
 use style::properties::animated_properties::compare_property_priority;
@@ -944,16 +944,35 @@ pub extern "C" fn Servo_StyleSet_MediumF
 
     // We'd like to return `OriginFlags` here, but bindgen bitfield enums don't
     // work as return values with the Linux 32-bit ABI at the moment because
     // they wrap the value in a struct, so for now just unwrap it.
     OriginFlags::from(origins_in_which_rules_changed).0
 }
 
 #[no_mangle]
+pub extern "C" fn Servo_StyleSet_SetDevice(
+    raw_data: RawServoStyleSetBorrowed,
+    pres_context: RawGeckoPresContextOwned
+) -> u8 {
+    let global_style_data = &*GLOBAL_STYLE_DATA;
+    let guard = global_style_data.shared_lock.read();
+
+    let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
+    let device = Device::new(pres_context);
+    let origins_in_which_rules_changed =
+        data.stylist.set_device(device, &guard);
+
+    // We'd like to return `OriginFlags` here, but bindgen bitfield enums don't
+    // work as return values with the Linux 32-bit ABI at the moment because
+    // they wrap the value in a struct, so for now just unwrap it.
+    OriginFlags::from(origins_in_which_rules_changed).0
+}
+
+#[no_mangle]
 pub extern "C" fn Servo_StyleSet_PrependStyleSheet(
     raw_data: RawServoStyleSetBorrowed,
     sheet: *const ServoStyleSheet,
 ) {
     let global_style_data = &*GLOBAL_STYLE_DATA;
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
     let data = &mut *data;
     let guard = global_style_data.shared_lock.read();