diff --git a/components/style/counter_style.rs b/components/style/counter_style.rs
index 0a3581a99b9..88de60751f7 100644
--- a/components/style/counter_style.rs
+++ b/components/style/counter_style.rs
@@ -193,8 +193,11 @@ counter_style_descriptors! {
         Fallback(CustomIdent(Atom::from("decimal")))
     }
 
-    /// https://drafts.csswg.org/css-counter-styles/#counter-style-symbols
+    /// https://drafts.csswg.org/css-counter-styles/#descdef-counter-style-symbols
     "symbols" symbols / eCSSCounterDesc_Symbols: Symbols = !
+
+    /// https://drafts.csswg.org/css-counter-styles/#descdef-counter-style-additive-symbols
+    "additive-symbols" additive_symbols / eCSSCounterDesc_AdditiveSymbols: Vec<AdditiveSymbol> = !
 }
 
 /// https://drafts.csswg.org/css-counter-styles/#counter-style-system
@@ -400,8 +403,7 @@ impl Parse for Pad {
 
 impl ToCss for Pad {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        write!(dest, "{}", self.0)?;
-        dest.write_char(' ')?;
+        write!(dest, "{} ", self.0)?;
         self.1.to_css(dest)
     }
 }
@@ -422,7 +424,7 @@ impl ToCss for Fallback {
     }
 }
 
-/// https://drafts.csswg.org/css-counter-styles/#counter-style-symbols
+/// https://drafts.csswg.org/css-counter-styles/#descdef-counter-style-symbols
 #[derive(Debug, Clone)]
 pub struct Symbols(pub Vec<Symbol>);
 
@@ -455,3 +457,31 @@ impl ToCss for Symbols {
         Ok(())
     }
 }
+
+/// https://drafts.csswg.org/css-counter-styles/#descdef-counter-style-additive-symbols
+#[derive(Debug, Clone)]
+pub struct AdditiveSymbol {
+    value: i32,
+    symbol: Symbol,
+}
+
+impl OneOrMoreCommaSeparated for AdditiveSymbol {}
+
+impl Parse for AdditiveSymbol {
+    fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        let value = input.try(|input| input.expect_integer());
+        let symbol = Symbol::parse(context, input)?;
+        let value = value.or_else(|()| input.expect_integer())?;
+        Ok(AdditiveSymbol {
+            value: value,
+            symbol: symbol,
+        })
+    }
+}
+
+impl ToCss for AdditiveSymbol {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+        write!(dest, "{} ", self.value)?;
+        self.symbol.to_css(dest)
+    }
+}
diff --git a/components/style/gecko/rules.rs b/components/style/gecko/rules.rs
index e36b1a5c724..d9e27faf683 100644
--- a/components/style/gecko/rules.rs
+++ b/components/style/gecko/rules.rs
@@ -234,3 +234,10 @@ impl ToNsCssValue for counter_style::Symbols {
         // FIXME: add bindings for nsCSSValueList
     }
 }
+
+impl ToNsCssValue for Vec<counter_style::AdditiveSymbol> {
+    fn convert(&self, _nscssvalue: &mut nsCSSValue) {
+        debug_assert!(!self.is_empty());
+        // FIXME: add bindings for nsCSSValuePairList
+    }
+}