diff --git a/components/style/element_state.rs b/components/style/element_state.rs
index d8b4d680377..dbc04ff0fdc 100644
--- a/components/style/element_state.rs
+++ b/components/style/element_state.rs
@@ -138,3 +138,16 @@ bitflags! {
         const IN_AUTOFILL_PREVIEW_STATE = 1 << 51,
     }
 }
+
+bitflags! {
+    /// Event-based document states.
+    ///
+    /// NB: Is important for this to remain in sync with Gecko's
+    /// dom/base/nsIDocument.h.
+    pub flags DocumentState: u64 {
+        /// RTL locale: specific to the XUL localedir attribute
+        const NS_DOCUMENT_STATE_RTL_LOCALE = 1 << 0,
+        /// Window activation status
+        const NS_DOCUMENT_STATE_WINDOW_INACTIVE = 1 << 1,
+    }
+}
diff --git a/components/style/gecko/generated/bindings.rs b/components/style/gecko/generated/bindings.rs
index 3c0d2d94506..6d7f36e22e3 100644
--- a/components/style/gecko/generated/bindings.rs
+++ b/components/style/gecko/generated/bindings.rs
@@ -219,6 +219,7 @@ use gecko_bindings::structs::nsStyleTransformMatrix::MatrixTransformOperator;
 unsafe impl Send for nsStyleTransformMatrix::MatrixTransformOperator {}
 unsafe impl Sync for nsStyleTransformMatrix::MatrixTransformOperator {}
 use gecko_bindings::structs::RawGeckoGfxMatrix4x4;
+use gecko_bindings::structs::EventStates;
 pub type nsTArrayBorrowed_uintptr_t<'a> = &'a mut ::gecko_bindings::structs::nsTArray<usize>;
 pub type RawServoStyleSetOwned = ::gecko_bindings::sugar::ownership::Owned<RawServoStyleSet>;
 pub type RawServoStyleSetOwnedOrNull = ::gecko_bindings::sugar::ownership::OwnedOrNull<RawServoStyleSet>;
@@ -586,6 +587,10 @@ extern "C" {
     pub fn Gecko_GetDocumentLWTheme(aDocument: *const nsIDocument)
      -> nsIDocument_DocumentTheme;
 }
+extern "C" {
+    pub fn Gecko_GetDocumentState(aDocument: *const nsIDocument)
+     -> EventStates;
+}
 extern "C" {
     pub fn Gecko_AtomAttrValue(element: RawGeckoElementBorrowed,
                                attribute: *mut nsIAtom) -> *mut nsIAtom;
diff --git a/components/style/gecko/non_ts_pseudo_class_list.rs b/components/style/gecko/non_ts_pseudo_class_list.rs
index 92da05b5b64..17ffd0e7046 100644
--- a/components/style/gecko/non_ts_pseudo_class_list.rs
+++ b/components/style/gecko/non_ts_pseudo_class_list.rs
@@ -26,8 +26,6 @@
  *
  * Pending pseudo-classes:
  *
- *  :-moz-window-inactive.
- *
  *  :scope -> <style scoped>, pending discussion.
  *
  * This follows the order defined in layout/style/nsCSSPseudoClassList.h when
@@ -116,6 +114,7 @@ macro_rules! apply_non_ts_list {
                 ("-moz-lwtheme", MozLWTheme, mozLWTheme, _, _),
                 ("-moz-lwtheme-brighttext", MozLWThemeBrightText, mozLWThemeBrightText, _, _),
                 ("-moz-lwtheme-darktext", MozLWThemeDarkText, mozLWThemeDarkText, _, _),
+                ("-moz-window-inactive", MozWindowInactive, mozWindowInactive, _, _),
             ],
             string: [
                 ("-moz-system-metric", MozSystemMetric, mozSystemMetric, _, PSEUDO_CLASS_INTERNAL),
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index f0ffb20127f..1602c8a1d06 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -22,7 +22,7 @@ use context::{QuirksMode, SharedStyleContext, UpdateAnimationsTasks};
 use data::ElementData;
 use dom::{self, DescendantsBit, LayoutIterator, NodeInfo, TElement, TNode, UnsafeNode};
 use dom::{OpaqueNode, PresentationalHintsSynthesizer};
-use element_state::ElementState;
+use element_state::{ElementState, DocumentState, NS_DOCUMENT_STATE_WINDOW_INACTIVE};
 use error_reporting::create_error_reporter;
 use font_metrics::{FontMetrics, FontMetricsProvider, FontMetricsQueryResult};
 use gecko::data::PerDocumentStyleData;
@@ -31,7 +31,7 @@ use gecko::selector_parser::{SelectorImpl, NonTSPseudoClass, PseudoElement};
 use gecko::snapshot_helpers;
 use gecko_bindings::bindings;
 use gecko_bindings::bindings::{Gecko_ConstructStyleChildrenIterator, Gecko_DestroyStyleChildrenIterator};
-use gecko_bindings::bindings::{Gecko_ElementState, Gecko_GetDocumentLWTheme};
+use gecko_bindings::bindings::{Gecko_ElementState, Gecko_GetDocumentLWTheme, Gecko_GetDocumentState};
 use gecko_bindings::bindings::{Gecko_GetLastChild, Gecko_GetNextStyleChild};
 use gecko_bindings::bindings::{Gecko_IsRootElement, Gecko_MatchesElement, Gecko_Namespace};
 use gecko_bindings::bindings::{Gecko_SetNodeFlags, Gecko_UnsetNodeFlags};
@@ -652,6 +652,14 @@ impl<'le> GeckoElement<'le> {
         unsafe { Gecko_ElementState(self.0) }
     }
 
+    fn document_state(&self) -> DocumentState {
+        let node = self.as_node();
+        unsafe {
+            let states = Gecko_GetDocumentState(node.owner_doc());
+            DocumentState::from_bits_truncate(states.mStates)
+        }
+    }
+
     #[inline]
     fn may_have_class(&self) -> bool {
         self.as_node().get_bool_flag(nsINode_BooleanFlag::ElementMayHaveClass)
@@ -1758,6 +1766,9 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             NonTSPseudoClass::MozLWThemeDarkText => {
                 self.get_document_theme() == DocumentTheme::Doc_Theme_Dark
             }
+            NonTSPseudoClass::MozWindowInactive => {
+                self.document_state().contains(NS_DOCUMENT_STATE_WINDOW_INACTIVE)
+            }
             NonTSPseudoClass::MozPlaceholder => false,
             NonTSPseudoClass::MozAny(ref sels) => {
                 let old_value = context.hover_active_quirk_disabled;