style: Avoid serializing the counter type in counter functions if it's decimal.

See https://github.com/w3c/web-platform-tests/pull/9862 and such.

components/style/values/specified/counters.rs

@@ -76,9 +76,22 @@ fn parse_counters<'i, 't>(
 
 #[cfg(feature = "servo")]
 type CounterStyleType = ListStyleType;
+
 #[cfg(feature = "gecko")]
 type CounterStyleType = CounterStyleOrNone;
 
+#[cfg(feature = "servo")]
+#[inline]
+fn is_decimal(counter_type: &CounterStyleType) -> bool {
+    *counter_type == ListStyleType::Decimal
+}
+
+#[cfg(feature = "gecko")]
+#[inline]
+fn is_decimal(counter_type: &CounterStyleType) -> bool {
+    *counter_type == CounterStyleOrNone::decimal()
+}
+
 /// The specified value for the `content` property.
 ///
 /// https://drafts.csswg.org/css-content/#propdef-content
@@ -102,10 +115,10 @@ pub enum ContentItem {
     String(Box<str>),
     /// `counter(name, style)`.
     #[css(comma, function)]
-    Counter(CustomIdent, CounterStyleType),
+    Counter(CustomIdent, #[css(skip_if = "is_decimal")] CounterStyleType),
     /// `counters(name, separator, style)`.
     #[css(comma, function)]
-    Counters(CustomIdent, Box<str>, CounterStyleType),
+    Counters(CustomIdent, Box<str>, #[css(skip_if = "is_decimal")] CounterStyleType),
     /// `open-quote`.
     OpenQuote,
     /// `close-quote`.

tests/wpt/metadata/css/cssom/serialize-values.html.ini

@@ -98,9 +98,3 @@
   [widows: inherit]
     expected: FAIL
 
-  [content: counter(par-num)]
-    expected: FAIL
-
-  [content: counter(par-num, decimal)]
-    expected: FAIL
-