style: Cleanup a few shorthand properties.

Differential Revision: https://phabricator.services.mozilla.com/D17082

components/style/properties/helpers.mako.rs

@@ -793,6 +793,58 @@
     % endif
 </%def>
 
+// A shorthand of kind `<property-1> <property-2>?` where both properties have
+// the same type.
+<%def name="two_properties_shorthand(
+    name,
+    first_property,
+    second_property,
+    parser_function,
+    needs_context=True,
+    **kwargs
+)">
+<%call expr="self.shorthand(name, sub_properties=' '.join([first_property, second_property]), **kwargs)">
+    #[allow(unused_imports)]
+    use crate::parser::Parse;
+    use crate::values::specified;
+
+    pub fn parse_value<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Longhands, ParseError<'i>> {
+        let parse_one = |_c: &ParserContext, input: &mut Parser<'i, 't>| {
+            % if needs_context:
+            ${parser_function}(_c, input)
+            % else:
+            ${parser_function}(input)
+            % endif
+        };
+
+        let first = parse_one(context, input)?;
+        let second =
+            input.try(|input| parse_one(context, input)).unwrap_or_else(|_| first.clone());
+        Ok(expanded! {
+            ${to_rust_ident(first_property)}: first,
+            ${to_rust_ident(second_property)}: second,
+        })
+    }
+
+    impl<'a> ToCss for LonghandsToSerialize<'a>  {
+        fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
+            let first = &self.${to_rust_ident(first_property)};
+            let second = &self.${to_rust_ident(second_property)};
+
+            first.to_css(dest)?;
+            if first != second {
+                dest.write_str(" ")?;
+                second.to_css(dest)?;
+            }
+            Ok(())
+        }
+    }
+</%call>
+</%def>
+
 <%def name="four_sides_shorthand(name, sub_property_pattern, parser_function,
                                  needs_context=True, allow_quirks=False, **kwargs)">
     <% sub_properties=' '.join(sub_property_pattern % side for side in PHYSICAL_SIDES) %>