diff --git a/components/selectors/parser.rs b/components/selectors/parser.rs
index 2fc5ad02d39..ccb4b943226 100644
--- a/components/selectors/parser.rs
+++ b/components/selectors/parser.rs
@@ -276,6 +276,11 @@ pub trait Parser<'i> {
         false
     }
 
+    /// Whether to allow forgiving selector-list parsing.
+    fn allow_forgiving_selectors(&self) -> bool {
+        true
+    }
+
     /// Parses non-tree-structural pseudo-classes. Tree structural pseudo-classes,
     /// like `:first-child`, are built into this library.
     ///
@@ -400,7 +405,11 @@ impl<Impl: SelectorImpl> SelectorList<Impl> {
                 Ok(selector) => values.push(selector),
                 Err(err) => match recovery {
                     ParseErrorRecovery::DiscardList => return Err(err),
-                    ParseErrorRecovery::IgnoreInvalidSelector => {},
+                    ParseErrorRecovery::IgnoreInvalidSelector => {
+                        if !parser.allow_forgiving_selectors() {
+                            return Err(err);
+                        }
+                    },
                 },
             }
 
diff --git a/components/style/gecko/pseudo_element_definition.mako.rs b/components/style/gecko/pseudo_element_definition.mako.rs
index dd6d1aad227..eab97c61bea 100644
--- a/components/style/gecko/pseudo_element_definition.mako.rs
+++ b/components/style/gecko/pseudo_element_definition.mako.rs
@@ -184,7 +184,7 @@ impl PseudoElement {
     ///
     /// Returns `None` if the pseudo-element is not recognised.
     #[inline]
-    pub fn from_slice(name: &str) -> Option<Self> {
+    pub fn from_slice(name: &str, allow_unkown_webkit: bool) -> Option<Self> {
         // We don't need to support tree pseudos because functional
         // pseudo-elements needs arguments, and thus should be created
         // via other methods.
@@ -209,7 +209,7 @@ impl PseudoElement {
                     return PseudoElement::tree_pseudo_element(name, Box::new([]))
                 }
                 const WEBKIT_PREFIX: &str = "-webkit-";
-                if starts_with_ignore_ascii_case(name, WEBKIT_PREFIX) {
+                if allow_unkown_webkit && starts_with_ignore_ascii_case(name, WEBKIT_PREFIX) {
                     let part = string_as_ascii_lowercase(&name[WEBKIT_PREFIX.len()..]);
                     return Some(PseudoElement::UnknownWebkit(part.into()));
                 }
diff --git a/components/style/gecko/selector_parser.rs b/components/style/gecko/selector_parser.rs
index aee0ce7fe35..6c975862f19 100644
--- a/components/style/gecko/selector_parser.rs
+++ b/components/style/gecko/selector_parser.rs
@@ -320,6 +320,11 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         function.eq_ignore_ascii_case("-moz-any")
     }
 
+    #[inline]
+    fn allow_forgiving_selectors(&self) -> bool {
+        !self.for_supports_rule
+    }
+
     fn parse_non_ts_pseudo_class(
         &self,
         location: SourceLocation,
@@ -373,7 +378,8 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         location: SourceLocation,
         name: CowRcStr<'i>,
     ) -> Result<PseudoElement, ParseError<'i>> {
-        if let Some(pseudo) = PseudoElement::from_slice(&name) {
+        let allow_unkown_webkit = !self.for_supports_rule;
+        if let Some(pseudo) = PseudoElement::from_slice(&name, allow_unkown_webkit) {
             if self.is_pseudo_element_enabled(&pseudo) {
                 return Ok(pseudo);
             }
diff --git a/components/style/selector_parser.rs b/components/style/selector_parser.rs
index 1af4d6ff07a..c401fe431ce 100644
--- a/components/style/selector_parser.rs
+++ b/components/style/selector_parser.rs
@@ -47,6 +47,8 @@ pub struct SelectorParser<'a> {
     /// The extra URL data of the stylesheet, which is used to look up
     /// whether we are parsing a chrome:// URL style sheet.
     pub url_data: &'a UrlExtraData,
+    /// Whether we're parsing selectors for `@supports`
+    pub for_supports_rule: bool,
 }
 
 impl<'a> SelectorParser<'a> {
@@ -63,6 +65,7 @@ impl<'a> SelectorParser<'a> {
             stylesheet_origin: Origin::Author,
             namespaces: &namespaces,
             url_data,
+            for_supports_rule: false,
         };
         let mut input = ParserInput::new(input);
         SelectorList::parse(&parser, &mut CssParser::new(&mut input))
diff --git a/components/style/stylesheets/rule_parser.rs b/components/style/stylesheets/rule_parser.rs
index 646a7dbbfd2..9cc2696463b 100644
--- a/components/style/stylesheets/rule_parser.rs
+++ b/components/style/stylesheets/rule_parser.rs
@@ -785,6 +785,7 @@ impl<'a, 'b, 'i> QualifiedRuleParser<'i> for NestedRuleParser<'a, 'b> {
             stylesheet_origin: self.context.stylesheet_origin,
             namespaces: self.namespaces,
             url_data: self.context.url_data,
+            for_supports_rule: false,
         };
         let selectors = SelectorList::parse(&selector_parser, input)?;
         if self.context.error_reporting_enabled() {
diff --git a/components/style/stylesheets/supports_rule.rs b/components/style/stylesheets/supports_rule.rs
index 0cc4c6746b9..071509bd181 100644
--- a/components/style/stylesheets/supports_rule.rs
+++ b/components/style/stylesheets/supports_rule.rs
@@ -385,29 +385,12 @@ impl RawSelector {
                     namespaces,
                     stylesheet_origin: context.stylesheet_origin,
                     url_data: context.url_data,
+                    for_supports_rule: true,
                 };
 
-                #[allow(unused_variables)]
-                let selector = Selector::<SelectorImpl>::parse(&parser, input)
+                Selector::<SelectorImpl>::parse(&parser, input)
                     .map_err(|_| input.new_custom_error(()))?;
 
-                #[cfg(feature = "gecko")]
-                {
-                    use crate::selector_parser::PseudoElement;
-                    use selectors::parser::Component;
-
-                    let has_any_unknown_webkit_pseudo = selector.has_pseudo_element() &&
-                        selector.iter_raw_match_order().any(|component| {
-                            matches!(
-                                *component,
-                                Component::PseudoElement(PseudoElement::UnknownWebkit(..))
-                            )
-                        });
-                    if has_any_unknown_webkit_pseudo {
-                        return Err(input.new_custom_error(()));
-                    }
-                }
-
                 Ok(())
             })
             .is_ok()