Issue #15977: Serialize counter-increment when there are no properties

components/style/properties/longhand/counters.mako.rs

@@ -269,6 +269,10 @@
 
     impl ToCss for SpecifiedValue {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+            if self.0.is_empty() {
+                return dest.write_str("none");
+            }
+
             let mut first = true;
             for pair in &self.0 {
                 if !first {
@@ -278,6 +282,7 @@
                 try!(serialize_identifier(&pair.0, dest));
                 try!(write!(dest, " {}", pair.1));
             }
+
             Ok(())
         }
     }