style: Implement the margin-block/inline shorthands.

Bug: 1519944
Reviewed-by: emilio

components/style/properties/shorthands/margin.mako.rs

@@ -8,3 +8,46 @@ ${helpers.four_sides_shorthand("margin", "margin-%s", "specified::LengthPercenta
                                spec="https://drafts.csswg.org/css-box/#propdef-margin",
                                allowed_in_page_rule=True,
                                allow_quirks=True)}
+
+% for axis in ["block", "inline"]:
+    <%
+        spec = "https://drafts.csswg.org/css-logical/#propdef-margin-%s" % axis
+    %>
+    <%helpers:shorthand
+        name="margin-${axis}"
+        sub_properties="${' '.join(
+            'margin-%s-%s' % (axis, side)
+            for side in ['start', 'end']
+        )}"
+        spec="${spec}">
+
+        use crate::parser::Parse;
+        use crate::values::specified::length::LengthPercentageOrAuto;
+        pub fn parse_value<'i, 't>(
+            context: &ParserContext,
+            input: &mut Parser<'i, 't>,
+        ) -> Result<Longhands, ParseError<'i>> {
+            let start_value = LengthPercentageOrAuto::parse(context, input)?;
+            let end_value =
+                input.try(|input| LengthPercentageOrAuto::parse(context, input)).unwrap_or_else(|_| start_value.clone());
+
+            Ok(expanded! {
+                margin_${axis}_start: start_value,
+                margin_${axis}_end: end_value,
+            })
+        }
+
+        impl<'a> ToCss for LonghandsToSerialize<'a>  {
+            fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
+                self.margin_${axis}_start.to_css(dest)?;
+
+                if self.margin_${axis}_end != self.margin_${axis}_start {
+                    dest.write_str(" ")?;
+                    self.margin_${axis}_end.to_css(dest)?;
+                }
+
+                Ok(())
+            }
+        }
+    </%helpers:shorthand>
+% endfor