diff --git a/components/selectors/parser.rs b/components/selectors/parser.rs
index bbc1dc4a246..6d15a741ca4 100644
--- a/components/selectors/parser.rs
+++ b/components/selectors/parser.rs
@@ -58,7 +58,7 @@ pub enum SelectorParseError<'i, T> {
     UnexpectedTokenInAttributeSelector,
     PseudoElementExpectedColon,
     PseudoElementExpectedIdent,
-    UnsupportedPseudoClass,
+    UnsupportedPseudoClassOrElement(CowRcStr<'i>),
     UnexpectedIdent(CowRcStr<'i>),
     ExpectedNamespace(CowRcStr<'i>),
     Custom(T),
@@ -136,7 +136,7 @@ pub trait Parser<'i> {
     fn parse_non_ts_pseudo_class(&self, name: CowRcStr<'i>)
                                  -> Result<<Self::Impl as SelectorImpl>::NonTSPseudoClass,
                                            ParseError<'i, SelectorParseError<'i, Self::Error>>> {
-        Err(ParseError::Custom(SelectorParseError::UnexpectedIdent(name)))
+        Err(ParseError::Custom(SelectorParseError::UnsupportedPseudoClassOrElement(name)))
     }
 
     fn parse_non_ts_functional_pseudo_class<'t>
@@ -144,20 +144,20 @@ pub trait Parser<'i> {
          -> Result<<Self::Impl as SelectorImpl>::NonTSPseudoClass,
                    ParseError<'i, SelectorParseError<'i, Self::Error>>>
     {
-        Err(ParseError::Custom(SelectorParseError::UnexpectedIdent(name)))
+        Err(ParseError::Custom(SelectorParseError::UnsupportedPseudoClassOrElement(name)))
     }
 
     fn parse_pseudo_element(&self, name: CowRcStr<'i>)
                             -> Result<<Self::Impl as SelectorImpl>::PseudoElement,
                                       ParseError<'i, SelectorParseError<'i, Self::Error>>> {
-        Err(ParseError::Custom(SelectorParseError::UnexpectedIdent(name)))
+        Err(ParseError::Custom(SelectorParseError::UnsupportedPseudoClassOrElement(name)))
     }
 
     fn parse_functional_pseudo_element<'t>
         (&self, name: CowRcStr<'i>, _arguments: &mut CssParser<'i, 't>)
          -> Result<<Self::Impl as SelectorImpl>::PseudoElement,
                    ParseError<'i, SelectorParseError<'i, Self::Error>>> {
-        Err(ParseError::Custom(SelectorParseError::UnexpectedIdent(name)))
+        Err(ParseError::Custom(SelectorParseError::UnsupportedPseudoClassOrElement(name)))
     }
 
     fn default_namespace(&self) -> Option<<Self::Impl as SelectorImpl>::NamespaceUrl> {
@@ -1503,9 +1503,9 @@ fn parse_compound_selector<'i, 't, P, E, Impl>(
                     };
 
                     let pseudo_class =
-                        P::parse_non_ts_pseudo_class(parser, name)?;
+                        P::parse_non_ts_pseudo_class(parser, name.clone())?;
                     if !p.supports_pseudo_class(&pseudo_class) {
-                        return Err(SelectorParseError::UnsupportedPseudoClass.into());
+                        return Err(SelectorParseError::UnsupportedPseudoClassOrElement(name).into());
                     }
                     state_selectors.push(Component::NonTSPseudoClass(pseudo_class));
                 }
diff --git a/components/style/gecko/selector_parser.rs b/components/style/gecko/selector_parser.rs
index 21c12305e6a..8000c0af5f4 100644
--- a/components/style/gecko/selector_parser.rs
+++ b/components/style/gecko/selector_parser.rs
@@ -308,7 +308,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
              keyword: [$(($k_css:expr, $k_name:ident, $k_gecko_type:tt, $k_state:tt, $k_flags:tt),)*]) => {
                 match_ignore_ascii_case! { &name,
                     $($css => NonTSPseudoClass::$name,)*
-                    _ => return Err(::selectors::parser::SelectorParseError::UnexpectedIdent(
+                    _ => return Err(::selectors::parser::SelectorParseError::UnsupportedPseudoClassOrElement(
                         name.clone()).into())
                 }
             }
@@ -317,7 +317,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         if self.is_pseudo_class_enabled(&pseudo_class) {
             Ok(pseudo_class)
         } else {
-            Err(SelectorParseError::UnexpectedIdent(name).into())
+            Err(SelectorParseError::UnsupportedPseudoClassOrElement(name).into())
         }
     }
 
@@ -354,7 +354,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
                         }
                         NonTSPseudoClass::MozAny(selectors.into_boxed_slice())
                     }
-                    _ => return Err(SelectorParseError::UnexpectedIdent(name.clone()).into())
+                    _ => return Err(SelectorParseError::UnsupportedPseudoClassOrElement(name.clone()).into())
                 }
             }
         }
@@ -362,13 +362,13 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         if self.is_pseudo_class_enabled(&pseudo_class) {
             Ok(pseudo_class)
         } else {
-            Err(SelectorParseError::UnexpectedIdent(name).into())
+            Err(SelectorParseError::UnsupportedPseudoClassOrElement(name).into())
         }
     }
 
     fn parse_pseudo_element(&self, name: CowRcStr<'i>) -> Result<PseudoElement, ParseError<'i>> {
         PseudoElement::from_slice(&name, self.in_user_agent_stylesheet())
-            .ok_or(SelectorParseError::UnexpectedIdent(name.clone()).into())
+            .ok_or(SelectorParseError::UnsupportedPseudoClassOrElement(name.clone()).into())
     }
 
     fn parse_functional_pseudo_element<'t>(&self, name: CowRcStr<'i>,
@@ -392,7 +392,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
                 return Ok(pseudo);
             }
         }
-        Err(SelectorParseError::UnexpectedIdent(name.clone()).into())
+        Err(SelectorParseError::UnsupportedPseudoClassOrElement(name.clone()).into())
     }
 
     fn default_namespace(&self) -> Option<Namespace> {
diff --git a/ports/geckolib/error_reporter.rs b/ports/geckolib/error_reporter.rs
index 583a9db6734..d487f649fef 100644
--- a/ports/geckolib/error_reporter.rs
+++ b/ports/geckolib/error_reporter.rs
@@ -250,6 +250,9 @@ fn extract_error_params<'a>(err: ParseError<'a>) -> Option<ErrorParams<'a>> {
                 PropertyDeclarationParseError::InvalidValue(property, Some(e))))) =>
             (Some(ErrorString::Snippet(property.into())), Some(extract_value_error_param(e))),
 
+        CssParseError::Custom(SelectorParseError::UnsupportedPseudoClassOrElement(p)) =>
+            (None, Some(ErrorString::Ident(p))),
+
         err => match extract_error_param(err) {
             Some(e) => (Some(e), None),
             None => return None,
@@ -325,8 +328,14 @@ impl<'a> ErrorHelpers<'a> for ContextualParseError<'a> {
                 _, CssParseError::Custom(SelectorParseError::Custom(
                 StyleParseError::UnexpectedTokenWithinNamespace(_)))) =>
                 (b"PEAtNSUnexpected\0", Action::Nothing),
-            ContextualParseError::InvalidRule(..) =>
-                (b"PEBadSelectorRSIgnored\0", Action::Nothing),
+            ContextualParseError::InvalidRule(_, ref err) => {
+                let prefix = match *err {
+                    CssParseError::Custom(SelectorParseError::UnsupportedPseudoClassOrElement(_)) =>
+                        Some(&b"PEPseudoSelUnknown\0"[..]),
+                    _ => None,
+                };
+                return (prefix, b"PEBadSelectorRSIgnored\0", Action::Nothing);
+            }
             ContextualParseError::UnsupportedRule(..) =>
                 (b"PEDeclDropped\0", Action::Nothing),
             ContextualParseError::UnsupportedViewportDescriptorDeclaration(..) |