style: Implement the inset-block/inline shorthands.

Bug: 1520229
Reviewed-by: emilio

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

@@ -762,3 +762,46 @@
         }
     }
 </%helpers:shorthand>
+
+% for axis in ["block", "inline"]:
+    <%
+        spec = "https://drafts.csswg.org/css-logical/#propdef-inset-%s" % axis
+    %>
+    <%helpers:shorthand
+        name="inset-${axis}"
+        sub_properties="${' '.join(
+            'inset-%s-%s' % (axis, side)
+            for side in ['start', 'end']
+        )}"
+        spec="${spec}">
+
+        use crate::parser::Parse;
+        use crate::values::specified::length::LengthPercentageOrAuto;
+        pub fn parse_value<'i, 't>(
+            context: &ParserContext,
+            input: &mut Parser<'i, 't>,
+        ) -> Result<Longhands, ParseError<'i>> {
+            let start_value = LengthPercentageOrAuto::parse(context, input)?;
+            let end_value =
+                input.try(|input| LengthPercentageOrAuto::parse(context, input)).unwrap_or_else(|_| start_value.clone());
+
+            Ok(expanded! {
+                inset_${axis}_start: start_value,
+                inset_${axis}_end: end_value,
+            })
+        }
+
+        impl<'a> ToCss for LonghandsToSerialize<'a>  {
+            fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
+                self.inset_${axis}_start.to_css(dest)?;
+
+                if self.inset_${axis}_end != self.inset_${axis}_start {
+                    dest.write_str(" ")?;
+                    self.inset_${axis}_end.to_css(dest)?;
+                }
+
+                Ok(())
+            }
+        }
+    </%helpers:shorthand>
+% endfor