diff --git a/components/selectors/builder.rs b/components/selectors/builder.rs
index 41b83b0c40e..352557735b3 100644
--- a/components/selectors/builder.rs
+++ b/components/selectors/builder.rs
@@ -17,7 +17,7 @@
 //! is non-trivial. This module encapsulates those details and presents an
 //! easy-to-use API for the parser.
 
-use crate::parser::{Combinator, Component, SelectorImpl};
+use crate::parser::{Combinator, Component, NonTSPseudoClass, SelectorImpl};
 use crate::sink::Push;
 use servo_arc::{Arc, HeaderWithLength, ThinArc};
 use smallvec::{self, SmallVec};
@@ -322,9 +322,13 @@ where
             Component::NthLastOfType(..) |
             Component::FirstOfType |
             Component::LastOfType |
-            Component::OnlyOfType |
-            Component::NonTSPseudoClass(..) => {
+            Component::OnlyOfType => {
                 specificity.class_like_selectors += 1;
+            }
+            Component::NonTSPseudoClass(ref pseudo) => {
+                if !pseudo.has_zero_specificity() {
+                    specificity.class_like_selectors += 1;
+                }
             },
             Component::ExplicitUniversalType |
             Component::ExplicitAnyNamespace |
diff --git a/components/selectors/parser.rs b/components/selectors/parser.rs
index 507121bbf97..92a5b039b90 100644
--- a/components/selectors/parser.rs
+++ b/components/selectors/parser.rs
@@ -52,6 +52,9 @@ pub trait NonTSPseudoClass: Sized + ToCss {
     ///
     /// https://drafts.csswg.org/selectors-4/#useraction-pseudos
     fn is_user_action_state(&self) -> bool;
+
+    /// Whether this pseudo-class has zero specificity.
+    fn has_zero_specificity(&self) -> bool;
 }
 
 /// Returns a Cow::Borrowed if `s` is already ASCII lowercase, and a
@@ -2336,6 +2339,11 @@ pub mod tests {
         fn is_user_action_state(&self) -> bool {
             self.is_active_or_hover()
         }
+
+        #[inline]
+        fn has_zero_specificity(&self) -> bool {
+            false
+        }
     }
 
     impl ToCss for PseudoClass {
diff --git a/components/style/gecko/non_ts_pseudo_class_list.rs b/components/style/gecko/non_ts_pseudo_class_list.rs
index f38757189d0..eca60fdc586 100644
--- a/components/style/gecko/non_ts_pseudo_class_list.rs
+++ b/components/style/gecko/non_ts_pseudo_class_list.rs
@@ -94,6 +94,7 @@ macro_rules! apply_non_ts_list {
                 ("-moz-last-node", MozLastNode, lastNode, _, _),
                 ("-moz-only-whitespace", MozOnlyWhitespace, mozOnlyWhitespace, _, _),
                 ("-moz-native-anonymous", MozNativeAnonymous, mozNativeAnonymous, _, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
+                ("-moz-native-anonymous-no-specificity", MozNativeAnonymousNoSpecificity, mozNativeAnonymousNoSpecificity, _, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
                 ("-moz-use-shadow-tree-root", MozUseShadowTreeRoot, mozUseShadowTreeRoot, _, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
                 ("-moz-is-html", MozIsHTML, mozIsHTML, _, _),
                 ("-moz-placeholder", MozPlaceholder, mozPlaceholder, _, _),
diff --git a/components/style/gecko/selector_parser.rs b/components/style/gecko/selector_parser.rs
index 2aad41adc9c..fec3c9247c3 100644
--- a/components/style/gecko/selector_parser.rs
+++ b/components/style/gecko/selector_parser.rs
@@ -233,6 +233,9 @@ impl NonTSPseudoClass {
                       // across all the elements involved and the latter is already
                       // checked for by our caching precondtions.
                       NonTSPseudoClass::MozIsHTML |
+                      // We prevent style sharing for NAC.
+                      NonTSPseudoClass::MozNativeAnonymous |
+                      NonTSPseudoClass::MozNativeAnonymousNoSpecificity |
                       // :-moz-placeholder is parsed but never matches.
                       NonTSPseudoClass::MozPlaceholder |
                       // :-moz-locale-dir and :-moz-window-inactive depend only on
@@ -275,6 +278,11 @@ impl ::selectors::parser::NonTSPseudoClass for NonTSPseudoClass {
             NonTSPseudoClass::Hover | NonTSPseudoClass::Active | NonTSPseudoClass::Focus
         )
     }
+
+    #[inline]
+    fn has_zero_specificity(&self) -> bool {
+        matches!(*self, NonTSPseudoClass::MozNativeAnonymousNoSpecificity)
+    }
 }
 
 /// The dummy struct we use to implement our selector parsing.
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index bf4da090646..e0079921072 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -2130,7 +2130,8 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
                 }
                 true
             },
-            NonTSPseudoClass::MozNativeAnonymous => self.is_in_native_anonymous_subtree(),
+            NonTSPseudoClass::MozNativeAnonymous |
+            NonTSPseudoClass::MozNativeAnonymousNoSpecificity => self.is_in_native_anonymous_subtree(),
             NonTSPseudoClass::MozUseShadowTreeRoot => self.is_root_of_use_element_shadow_tree(),
             NonTSPseudoClass::MozTableBorderNonzero => unsafe {
                 bindings::Gecko_IsTableBorderNonzero(self.0)
diff --git a/components/style/servo/selector_parser.rs b/components/style/servo/selector_parser.rs
index 977a6c59a85..942dcdba1bb 100644
--- a/components/style/servo/selector_parser.rs
+++ b/components/style/servo/selector_parser.rs
@@ -309,6 +309,11 @@ impl ::selectors::parser::NonTSPseudoClass for NonTSPseudoClass {
             NonTSPseudoClass::Active | NonTSPseudoClass::Hover | NonTSPseudoClass::Focus
         )
     }
+
+    #[inline]
+    fn has_zero_specificity(&self) -> bool {
+        false
+    }
 }
 
 impl ToCss for NonTSPseudoClass {