diff --git a/components/style/properties/helpers.mako.rs b/components/style/properties/helpers.mako.rs
index fa296a3046a..3e24a08103b 100644
--- a/components/style/properties/helpers.mako.rs
+++ b/components/style/properties/helpers.mako.rs
@@ -341,7 +341,11 @@
                             }
                             context.builder.put_${data.current_style_struct.name_lower}(s);
                         % else:
+                            % if property.boxed:
+                            let computed = (**specified_value).to_computed_value(context);
+                            % else:
                             let computed = specified_value.to_computed_value(context);
+                            % endif
                             % if property.ident == "font_size":
                                  longhands::font_size::cascade_specified_font_size(
                                      context,
diff --git a/components/style/properties/helpers/animated_properties.mako.rs b/components/style/properties/helpers/animated_properties.mako.rs
index bb0fd657b5d..278dcf2138d 100644
--- a/components/style/properties/helpers/animated_properties.mako.rs
+++ b/components/style/properties/helpers/animated_properties.mako.rs
@@ -544,7 +544,11 @@ impl AnimationValue {
                     longhands::system_font::resolve_system_font(sf, context);
                 }
             % endif
+            % if prop.boxed:
+            let computed = (**val).to_computed_value(context);
+            % else:
             let computed = val.to_computed_value(context);
+            % endif
             Some(AnimationValue::${prop.camel_case}(
             % if prop.is_animatable_with_computed_value:
                 computed
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index 7df0886ccd5..a0ebfda3171 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -408,7 +408,7 @@ impl ToComputedValue for specified::LengthOrPercentage {
                 LengthOrPercentage::Percentage(value)
             }
             specified::LengthOrPercentage::Calc(ref calc) => {
-                LengthOrPercentage::Calc(calc.to_computed_value(context))
+                LengthOrPercentage::Calc((**calc).to_computed_value(context))
             }
         }
     }
@@ -502,7 +502,7 @@ impl ToComputedValue for specified::LengthOrPercentageOrAuto {
                 LengthOrPercentageOrAuto::Auto
             }
             specified::LengthOrPercentageOrAuto::Calc(ref calc) => {
-                LengthOrPercentageOrAuto::Calc(calc.to_computed_value(context))
+                LengthOrPercentageOrAuto::Calc((**calc).to_computed_value(context))
             }
         }
     }
@@ -591,7 +591,7 @@ impl ToComputedValue for specified::LengthOrPercentageOrNone {
                 LengthOrPercentageOrNone::Percentage(value)
             }
             specified::LengthOrPercentageOrNone::Calc(ref calc) => {
-                LengthOrPercentageOrNone::Calc(calc.to_computed_value(context))
+                LengthOrPercentageOrNone::Calc((**calc).to_computed_value(context))
             }
             specified::LengthOrPercentageOrNone::None => {
                 LengthOrPercentageOrNone::None
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index 94cc1706c4a..aedb8ca31f2 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -296,6 +296,22 @@ impl<T> ToComputedValue for Vec<T>
     }
 }
 
+impl<T> ToComputedValue for Box<T>
+    where T: ToComputedValue
+{
+    type ComputedValue = Box<<T as ToComputedValue>::ComputedValue>;
+
+    #[inline]
+    fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
+        Box::new(T::to_computed_value(self, context))
+    }
+
+    #[inline]
+    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
+        Box::new(T::from_computed_value(computed))
+    }
+}
+
 impl<T> ToComputedValue for Box<[T]>
     where T: ToComputedValue
 {