diff --git a/components/style/element_state.rs b/components/style/element_state.rs
index 190c1cc3eb5..3fe94f3e882 100644
--- a/components/style/element_state.rs
+++ b/components/style/element_state.rs
@@ -141,6 +141,10 @@ bitflags! {
         ///
         /// https://drafts.csswg.org/selectors-4/#the-focus-visible-pseudo
         const IN_FOCUS_VISIBLE_STATE = 1 << 52;
+        /// State that dialog element is modal, for centered alignment
+        ///
+        /// https://html.spec.whatwg.org/#centered-alignment
+        const IN_MODAL_DIALOG_STATE = 1 << 53;
     }
 }
 
diff --git a/components/style/gecko/non_ts_pseudo_class_list.rs b/components/style/gecko/non_ts_pseudo_class_list.rs
index b82818b9bae..7b737f4d9cf 100644
--- a/components/style/gecko/non_ts_pseudo_class_list.rs
+++ b/components/style/gecko/non_ts_pseudo_class_list.rs
@@ -50,6 +50,7 @@ macro_rules! apply_non_ts_list {
                 ("-moz-devtools-highlighted", MozDevtoolsHighlighted, IN_DEVTOOLS_HIGHLIGHTED_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
                 ("-moz-styleeditor-transitioning", MozStyleeditorTransitioning, IN_STYLEEDITOR_TRANSITIONING_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
                 ("fullscreen", Fullscreen, IN_FULLSCREEN_STATE, _),
+                ("-moz-modal-dialog", MozModalDialog, IN_MODAL_DIALOG_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
                 // TODO(emilio): This is inconsistently named (the capital R).
                 ("-moz-focusring", MozFocusRing, IN_FOCUSRING_STATE, _),
                 ("-moz-broken", MozBroken, IN_BROKEN_STATE, _),
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 0fbaf5f0aba..62af71f212b 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -2062,6 +2062,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             NonTSPseudoClass::MozDirAttrRTL |
             NonTSPseudoClass::MozDirAttrLikeAuto |
             NonTSPseudoClass::MozAutofill |
+            NonTSPseudoClass::MozModalDialog |
             NonTSPseudoClass::Active |
             NonTSPseudoClass::Hover |
             NonTSPseudoClass::MozAutofillPreview => {