diff --git a/components/style/gecko/selector_parser.rs b/components/style/gecko/selector_parser.rs
index adb410908b9..a46068d923a 100644
--- a/components/style/gecko/selector_parser.rs
+++ b/components/style/gecko/selector_parser.rs
@@ -354,7 +354,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
 
     #[inline]
     fn parse_is_and_where(&self) -> bool {
-        static_prefs::pref!("layout.css.is-where-selectors.enabled")
+        self.in_user_agent_stylesheet() || static_prefs::pref!("layout.css.is-where-selectors.enabled")
     }
 
     #[inline]