layout: Implement outline-offset per CSS-UI-3 § 5.5.

components/style/properties/common_types.rs

@@ -933,6 +933,14 @@ pub mod computed {
             }
         }
     }
+
+    #[allow(non_snake_case)]
+    #[inline]
+    pub fn compute_Length(value: specified::Length, context: &Context) -> Au {
+        compute_Au(value, context)
+    }
+
+    pub type Length = Au;
 }
 
 pub fn parse_url(input: &str, base_url: &Url) -> Url {

components/style/properties/mod.rs.mako

@@ -381,6 +381,8 @@ pub mod longhands {
         }
     </%self:longhand>
 
+    ${predefined_type("outline-offset", "Length", "Au(0)")}
+
     ${new_style_struct("PositionOffsets", is_inherited=False)}
 
     % for side in ["top", "right", "bottom", "left"]: