Move Animatable::get_zero_value to ToAnimatedZero::to_animated_zero

components/style/macros.rs

@@ -112,5 +112,10 @@ macro_rules! define_keyword_type {
         impl $crate::values::computed::ComputedValueAsSpecified for $name {}
         impl $crate::values::animated::AnimatedValueAsComputed for $name {}
         no_viewport_percentage!($name);
+
+        impl $crate::values::animated::ToAnimatedZero for $name {
+            #[inline]
+            fn to_animated_zero(&self) -> Result<Self, ()> { Ok($name) }
+        }
     };
 }

components/style/properties/helpers.mako.rs

@@ -129,6 +129,8 @@
 
             % if animation_value_type == "ComputedValue":
                 use properties::animated_properties::Animatable;
+                use values::animated::ToAnimatedZero;
+
                 impl Animatable for T {
                     fn add_weighted(&self, other: &Self, self_portion: f64, other_portion: f64)
                         -> Result<Self, ()> {
@@ -149,6 +151,11 @@
                         self.0.compute_squared_distance(&other.0)
                     }
                 }
+
+                impl ToAnimatedZero for T {
+                    #[inline]
+                    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+                }
             % endif
 
             pub type Iter<'a, 'cx, 'cx_a> = ComputedVecIter<'a, 'cx, 'cx_a, super::single_value::SpecifiedValue>;

components/style/properties/helpers/animated_properties.mako.rs

@@ -33,7 +33,7 @@ use super::ComputedValuesInner;
 #[cfg(any(feature = "gecko", feature = "testing"))]
 use values::Auto;
 use values::{CSSFloat, CustomIdent, Either};
-use values::animated::ToAnimatedValue;
+use values::animated::{ToAnimatedValue, ToAnimatedZero};
 use values::animated::effects::BoxShadowList as AnimatedBoxShadowList;
 use values::animated::effects::Filter as AnimatedFilter;
 use values::animated::effects::FilterList as AnimatedFilterList;
@@ -77,16 +77,6 @@ pub trait Animatable: Sized {
         self.add_weighted(other, count as f64, 1.0)
     }
 
-    /// Returns a value that, when added with an underlying value, will produce the underlying
-    /// value. This is used for SMIL animation's "by-animation" where SMIL first interpolates from
-    /// the zero value to the 'by' value, and then adds the result to the underlying value.
-    ///
-    /// This is not the necessarily the same as the initial value of a property. For example, the
-    /// initial value of 'stroke-width' is 1, but the zero value is 0, since adding 1 to the
-    /// underlying value will not produce the underlying value.
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> { Err(()) }
-
     /// Compute distance between a value and another for a given property.
     fn compute_distance(&self, _other: &Self) -> Result<f64, ()>  { Err(()) }
 
@@ -745,19 +735,6 @@ impl Animatable for AnimationValue {
         }
     }
 
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        match *self {
-            % for prop in data.longhands:
-            % if prop.animatable and prop.animation_value_type != "discrete":
-            AnimationValue::${prop.camel_case}(ref base) => {
-                Ok(AnimationValue::${prop.camel_case}(base.get_zero_value()?))
-            },
-            % endif
-            % endfor
-            _ => Err(()),
-        }
-    }
-
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (self, other) {
             % for prop in data.longhands:
@@ -783,6 +760,22 @@ impl Animatable for AnimationValue {
     }
 }
 
+impl ToAnimatedZero for AnimationValue {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        match *self {
+            % for prop in data.longhands:
+            % if prop.animatable and prop.animation_value_type != "discrete":
+            AnimationValue::${prop.camel_case}(ref base) => {
+                Ok(AnimationValue::${prop.camel_case}(base.to_animated_zero()?))
+            },
+            % endif
+            % endfor
+            _ => Err(()),
+        }
+    }
+}
+
 impl RepeatableListAnimatable for LengthOrPercentage {}
 impl RepeatableListAnimatable for Either<f32, LengthOrPercentage> {}
 
@@ -832,9 +825,6 @@ impl Animatable for Au {
         Ok(Au((self.0 as f64 * self_portion + other.0 as f64 * other_portion).round() as i32))
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> { Ok(Au(0)) }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         self.0.compute_distance(&other.0)
@@ -885,9 +875,6 @@ impl Animatable for f32 {
         Ok((*self as f64 * self_portion + *other as f64 * other_portion) as f32)
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> { Ok(0.) }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         Ok((*self - *other).abs() as f64)
@@ -901,9 +888,6 @@ impl Animatable for f64 {
         Ok(*self * self_portion + *other * other_portion)
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> { Ok(0.) }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         Ok((*self - *other).abs())
@@ -917,9 +901,6 @@ impl Animatable for i32 {
         Ok((*self as f64 * self_portion + *other as f64 * other_portion).round() as i32)
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> { Ok(0) }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         Ok((*self - *other).abs() as f64)
@@ -954,15 +935,19 @@ impl Animatable for Percentage {
         Ok(Percentage((self.0 as f64 * self_portion + other.0 as f64 * other_portion) as f32))
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> { Ok(Percentage(0.)) }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         Ok((self.0 as f64 - other.0 as f64).abs())
     }
 }
 
+impl ToAnimatedZero for Percentage {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(Percentage(0.))
+    }
+}
+
 /// https://drafts.csswg.org/css-transitions/#animtype-visibility
 impl Animatable for Visibility {
     #[inline]
@@ -988,6 +973,13 @@ impl Animatable for Visibility {
     }
 }
 
+impl ToAnimatedZero for Visibility {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Err(())
+    }
+}
+
 impl<T: Animatable + Copy> Animatable for Size2D<T> {
     #[inline]
     fn add_weighted(&self, other: &Self, self_portion: f64, other_portion: f64) -> Result<Self, ()> {
@@ -1026,6 +1018,11 @@ impl Animatable for BorderCornerRadius {
     }
 }
 
+impl ToAnimatedZero for BorderCornerRadius {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}
+
 /// https://drafts.csswg.org/css-transitions/#animtype-length
 impl Animatable for VerticalAlign {
     #[inline]
@@ -1053,6 +1050,11 @@ impl Animatable for VerticalAlign {
     }
 }
 
+impl ToAnimatedZero for VerticalAlign {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}
+
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Animatable for CalcLengthOrPercentage {
     #[inline]
@@ -1123,11 +1125,6 @@ impl Animatable for LengthOrPercentage {
         }
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        Ok(LengthOrPercentage::zero())
-    }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (*self, *other) {
@@ -1171,6 +1168,13 @@ impl Animatable for LengthOrPercentage {
     }
 }
 
+impl ToAnimatedZero for LengthOrPercentage {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(LengthOrPercentage::zero())
+    }
+}
+
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Animatable for LengthOrPercentageOrAuto {
     #[inline]
@@ -1200,18 +1204,6 @@ impl Animatable for LengthOrPercentageOrAuto {
         }
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        match *self {
-            LengthOrPercentageOrAuto::Length(_) |
-            LengthOrPercentageOrAuto::Percentage(_) |
-            LengthOrPercentageOrAuto::Calc(_) => {
-                Ok(LengthOrPercentageOrAuto::Length(Au(0)))
-            },
-            LengthOrPercentageOrAuto::Auto => Err(()),
-        }
-    }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (*self, *other) {
@@ -1260,6 +1252,20 @@ impl Animatable for LengthOrPercentageOrAuto {
     }
 }
 
+impl ToAnimatedZero for LengthOrPercentageOrAuto {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        match *self {
+            LengthOrPercentageOrAuto::Length(_) |
+            LengthOrPercentageOrAuto::Percentage(_) |
+            LengthOrPercentageOrAuto::Calc(_) => {
+                Ok(LengthOrPercentageOrAuto::Length(Au(0)))
+            },
+            LengthOrPercentageOrAuto::Auto => Err(()),
+        }
+    }
+}
+
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Animatable for LengthOrPercentageOrNone {
     #[inline]
@@ -1289,18 +1295,6 @@ impl Animatable for LengthOrPercentageOrNone {
         }
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        match *self {
-            LengthOrPercentageOrNone::Length(_) |
-            LengthOrPercentageOrNone::Percentage(_) |
-            LengthOrPercentageOrNone::Calc(_) => {
-                Ok(LengthOrPercentageOrNone::Length(Au(0)))
-            },
-            LengthOrPercentageOrNone::None => Err(()),
-        }
-    }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (*self, *other) {
@@ -1322,6 +1316,20 @@ impl Animatable for LengthOrPercentageOrNone {
     }
 }
 
+impl ToAnimatedZero for LengthOrPercentageOrNone {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        match *self {
+            LengthOrPercentageOrNone::Length(_) |
+            LengthOrPercentageOrNone::Percentage(_) |
+            LengthOrPercentageOrNone::Calc(_) => {
+                Ok(LengthOrPercentageOrNone::Length(Au(0)))
+            },
+            LengthOrPercentageOrNone::None => Err(()),
+        }
+    }
+}
+
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Animatable for MozLength {
     #[inline]
@@ -1348,6 +1356,11 @@ impl Animatable for MozLength {
     }
 }
 
+impl ToAnimatedZero for MozLength {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}
+
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Animatable for MaxLength {
     #[inline]
@@ -1374,6 +1387,11 @@ impl Animatable for MaxLength {
     }
 }
 
+impl ToAnimatedZero for MaxLength {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}
+
 /// http://dev.w3.org/csswg/css-transitions/#animtype-font-weight
 impl Animatable for FontWeight {
     #[inline]
@@ -1386,9 +1404,6 @@ impl Animatable for FontWeight {
         Ok(FontWeight(weight as u16))
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> { Ok(FontWeight::normal()) }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         let a = self.0 as f64;
@@ -1397,6 +1412,13 @@ impl Animatable for FontWeight {
     }
 }
 
+impl ToAnimatedZero for FontWeight {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(FontWeight::normal())
+    }
+}
+
 /// https://drafts.csswg.org/css-fonts/#font-stretch-prop
 impl Animatable for FontStretch {
     #[inline]
@@ -1420,6 +1442,11 @@ impl Animatable for FontStretch {
     }
 }
 
+impl ToAnimatedZero for FontStretch {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}
+
 /// We should treat font stretch as real number in order to interpolate this property.
 /// https://drafts.csswg.org/css-fonts-3/#font-stretch-animation
 impl From<FontStretch> for f64 {
@@ -1460,14 +1487,6 @@ impl<H: Animatable, V: Animatable> Animatable for generic_position::Position<H,
         })
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        Ok(generic_position::Position {
-            horizontal: self.horizontal.get_zero_value()?,
-            vertical: self.vertical.get_zero_value()?,
-        })
-    }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         self.compute_squared_distance(other).map(|sd| sd.sqrt())
@@ -1480,6 +1499,20 @@ impl<H: Animatable, V: Animatable> Animatable for generic_position::Position<H,
     }
 }
 
+impl<H, V> ToAnimatedZero for generic_position::Position<H, V>
+where
+    H: ToAnimatedZero,
+    V: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(generic_position::Position {
+            horizontal: self.horizontal.to_animated_zero()?,
+            vertical: self.vertical.to_animated_zero()?,
+        })
+    }
+}
+
 impl<H, V> RepeatableListAnimatable for generic_position::Position<H, V>
     where H: RepeatableListAnimatable, V: RepeatableListAnimatable {}
 
@@ -1513,6 +1546,11 @@ impl Animatable for ClipRect {
     }
 }
 
+impl ToAnimatedZero for ClipRect {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}
+
 /// Check if it's possible to do a direct numerical interpolation
 /// between these two transform lists.
 /// http://dev.w3.org/csswg/css-transforms/#transform-transform-animation
@@ -2587,9 +2625,13 @@ impl Animatable for TransformList {
             }
         }
     }
+}
 
+impl ToAnimatedZero for TransformList {
     #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> { Ok(TransformList(None)) }
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(TransformList(None))
+    }
 }
 
 impl<T, U> Animatable for Either<T, U>
@@ -2611,18 +2653,6 @@ impl<T, U> Animatable for Either<T, U>
         }
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        match *self {
-            Either::First(ref this) => {
-                Ok(Either::First(this.get_zero_value()?))
-            },
-            Either::Second(ref this) => {
-                Ok(Either::Second(this.get_zero_value()?))
-            },
-        }
-    }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (self, other) {
@@ -2650,6 +2680,24 @@ impl<T, U> Animatable for Either<T, U>
     }
 }
 
+impl<A, B> ToAnimatedZero for Either<A, B>
+where
+    A: ToAnimatedZero,
+    B: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        match *self {
+            Either::First(ref first) => {
+                Ok(Either::First(first.to_animated_zero()?))
+            },
+            Either::Second(ref second) => {
+                Ok(Either::Second(second.to_animated_zero()?))
+            },
+        }
+    }
+}
+
 #[derive(Copy, Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 /// Unlike RGBA, each component value may exceed the range [0.0, 1.0].
@@ -2728,11 +2776,6 @@ impl Animatable for IntermediateRGBA {
         }
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        Ok(IntermediateRGBA::transparent())
-    }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         self.compute_squared_distance(other).map(|sq| sq.sqrt())
@@ -2757,6 +2800,13 @@ impl Animatable for IntermediateRGBA {
     }
 }
 
+impl ToAnimatedZero for IntermediateRGBA {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(IntermediateRGBA::transparent())
+    }
+}
+
 #[derive(Copy, Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
@@ -2895,6 +2945,11 @@ impl Animatable for IntermediateColor {
     }
 }
 
+impl ToAnimatedZero for IntermediateColor {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}
+
 /// Animatable SVGPaint
 pub type IntermediateSVGPaint = SVGPaint<IntermediateRGBA>;
 
@@ -2920,12 +2975,14 @@ impl Animatable for IntermediateSVGPaint {
         Ok(self.kind.compute_squared_distance(&other.kind)? +
             self.fallback.compute_squared_distance(&other.fallback)?)
     }
+}
 
+impl ToAnimatedZero for IntermediateSVGPaint {
     #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
+    fn to_animated_zero(&self) -> Result<Self, ()> {
         Ok(IntermediateSVGPaint {
-            kind: self.kind.get_zero_value()?,
-            fallback: self.fallback.and_then(|v| v.get_zero_value().ok()),
+            kind: self.kind.to_animated_zero()?,
+            fallback: self.fallback.and_then(|v| v.to_animated_zero().ok()),
         })
     }
 }
@@ -2958,12 +3015,14 @@ impl Animatable for IntermediateSVGPaintKind {
             _ => Err(())
         }
     }
+}
 
+impl ToAnimatedZero for IntermediateSVGPaintKind {
     #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
+    fn to_animated_zero(&self) -> Result<Self, ()> {
         match *self {
             SVGPaintKind::Color(ref color) => {
-                Ok(SVGPaintKind::Color(color.get_zero_value()?))
+                Ok(SVGPaintKind::Color(color.to_animated_zero()?))
             },
             SVGPaintKind::None |
             SVGPaintKind::ContextFill |

components/style/properties/longhand/font.mako.rs

@@ -1044,6 +1044,7 @@ ${helpers.single_keyword_system("font-variant-caps",
     pub mod computed_value {
         use properties::animated_properties::Animatable;
         use values::CSSFloat;
+        use values::animated::ToAnimatedZero;
 
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         #[derive(Copy, Clone, Debug, PartialEq, ToCss)]
@@ -1081,6 +1082,11 @@ ${helpers.single_keyword_system("font-variant-caps",
                 }
             }
         }
+
+        impl ToAnimatedZero for T {
+            #[inline]
+            fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+        }
     }
 
     #[inline]

components/style/properties/longhand/inherited_table.mako.rs

@@ -28,6 +28,7 @@ ${helpers.single_keyword("caption-side", "top bottom",
     pub mod computed_value {
         use app_units::Au;
         use properties::animated_properties::Animatable;
+        use values::animated::ToAnimatedZero;
 
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         #[derive(Clone, Copy, Debug, PartialEq, ToCss)]
@@ -60,6 +61,11 @@ ${helpers.single_keyword("caption-side", "top bottom",
                    self.vertical.compute_squared_distance(&other.vertical)?)
             }
         }
+
+        impl ToAnimatedZero for T {
+            #[inline]
+            fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+        }
     }
 
     #[cfg_attr(feature = "servo", derive(HeapSizeOf))]

components/style/values/animated/effects.rs

@@ -11,7 +11,7 @@ use properties::longhands::text_shadow::computed_value::T as ComputedTextShadowL
 use std::cmp;
 #[cfg(not(feature = "gecko"))]
 use values::Impossible;
-use values::animated::ToAnimatedValue;
+use values::animated::{ToAnimatedValue, ToAnimatedZero};
 use values::computed::{Angle, Number};
 use values::computed::length::Length;
 use values::generics::effects::BoxShadow as GenericBoxShadow;
@@ -66,7 +66,7 @@ impl ToAnimatedValue for ComputedBoxShadowList {
 
 impl<S> Animatable for ShadowList<S>
 where
-    S: Animatable + Clone,
+    S: Animatable + Clone + ToAnimatedZero,
 {
     #[inline]
     fn add_weighted(
@@ -83,10 +83,10 @@ where
                     shadow.add_weighted(other, self_portion, other_portion)?
                 },
                 (Some(shadow), None) => {
-                    shadow.add_weighted(&shadow.get_zero_value()?, self_portion, other_portion)?
+                    shadow.add_weighted(&shadow.to_animated_zero()?, self_portion, other_portion)?
                 },
                 (None, Some(shadow)) => {
-                    shadow.get_zero_value()?.add_weighted(&shadow, self_portion, other_portion)?
+                    shadow.to_animated_zero()?.add_weighted(&shadow, self_portion, other_portion)?
                 },
                 (None, None) => unreachable!(),
             });
@@ -102,6 +102,13 @@ where
     }
 }
 
+impl<S> ToAnimatedZero for ShadowList<S> {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(ShadowList(vec![]))
+    }
+}
+
 impl ToAnimatedValue for ComputedTextShadowList {
     type AnimatedValue = TextShadowList;
 
@@ -134,15 +141,6 @@ impl Animatable for BoxShadow {
         })
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        Ok(BoxShadow {
-            base: self.base.get_zero_value()?,
-            spread: self.spread.get_zero_value()?,
-            inset: self.inset,
-        })
-    }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         self.compute_squared_distance(other).map(|sd| sd.sqrt())
@@ -160,6 +158,17 @@ impl Animatable for BoxShadow {
     }
 }
 
+impl ToAnimatedZero for BoxShadow {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(BoxShadow {
+            base: self.base.to_animated_zero()?,
+            spread: self.spread.to_animated_zero()?,
+            inset: self.inset,
+        })
+    }
+}
+
 impl ToAnimatedValue for ComputedFilterList {
     type AnimatedValue = FilterList;
 
@@ -188,6 +197,13 @@ impl ToAnimatedValue for ComputedFilterList {
     }
 }
 
+impl ToAnimatedZero for FilterList {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(FilterList(vec![]))
+    }
+}
+
 impl Animatable for SimpleShadow {
     #[inline]
     fn add_weighted(&self, other: &Self, self_portion: f64, other_portion: f64) -> Result<Self, ()> {
@@ -204,16 +220,6 @@ impl Animatable for SimpleShadow {
         })
     }
 
-    #[inline]
-    fn get_zero_value(&self) -> Result<Self, ()> {
-        Ok(SimpleShadow {
-            color: IntermediateColor::transparent(),
-            horizontal: self.horizontal.get_zero_value()?,
-            vertical: self.vertical.get_zero_value()?,
-            blur: self.blur.get_zero_value()?,
-        })
-    }
-
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         self.compute_squared_distance(other).map(|sd| sd.sqrt())
@@ -229,3 +235,15 @@ impl Animatable for SimpleShadow {
         )
     }
 }
+
+impl ToAnimatedZero for SimpleShadow {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(SimpleShadow {
+            color: IntermediateColor::transparent(),
+            horizontal: self.horizontal.to_animated_zero()?,
+            vertical: self.vertical.to_animated_zero()?,
+            blur: self.blur.to_animated_zero()?,
+        })
+    }
+}

components/style/values/animated/mod.rs

@@ -88,3 +88,34 @@ where
     }
 }
 
+/// Returns a value similar to `self` that represents zero.
+pub trait ToAnimatedZero: Sized {
+    /// Returns a value that, when added with an underlying value, will produce the underlying
+    /// value. This is used for SMIL animation's "by-animation" where SMIL first interpolates from
+    /// the zero value to the 'by' value, and then adds the result to the underlying value.
+    ///
+    /// This is not the necessarily the same as the initial value of a property. For example, the
+    /// initial value of 'stroke-width' is 1, but the zero value is 0, since adding 1 to the
+    /// underlying value will not produce the underlying value.
+    fn to_animated_zero(&self) -> Result<Self, ()>;
+}
+
+impl ToAnimatedZero for Au {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Ok(Au(0)) }
+}
+
+impl ToAnimatedZero for f32 {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Ok(0.) }
+}
+
+impl ToAnimatedZero for f64 {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Ok(0.) }
+}
+
+impl ToAnimatedZero for i32 {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Ok(0) }
+}

components/style/values/computed/background.rs

@@ -5,6 +5,7 @@
 //! Computed types for CSS values related to backgrounds.
 
 use properties::animated_properties::{Animatable, RepeatableListAnimatable};
+use values::animated::ToAnimatedZero;
 use values::computed::length::LengthOrPercentageOrAuto;
 use values::generics::background::BackgroundSize as GenericBackgroundSize;
 
@@ -50,3 +51,8 @@ impl Animatable for BackgroundSize {
         }
     }
 }
+
+impl ToAnimatedZero for BackgroundSize {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}

components/style/values/computed/text.rs

@@ -7,6 +7,7 @@
 use app_units::Au;
 use properties::animated_properties::Animatable;
 use values::{CSSInteger, CSSFloat};
+use values::animated::ToAnimatedZero;
 use values::computed::length::{Length, LengthOrPercentage};
 use values::generics::text::InitialLetter as GenericInitialLetter;
 use values::generics::text::LineHeight as GenericLineHeight;
@@ -61,3 +62,8 @@ impl Animatable for LineHeight {
         }
     }
 }
+
+impl ToAnimatedZero for LineHeight {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}

components/style/values/computed/transform.rs

@@ -5,6 +5,7 @@
 //! Computed types for CSS values that are related to transformations.
 
 use properties::animated_properties::Animatable;
+use values::animated::ToAnimatedZero;
 use values::computed::{Length, LengthOrPercentage, Number, Percentage};
 use values::generics::transform::TimingFunction as GenericTimingFunction;
 use values::generics::transform::TransformOrigin as GenericTransformOrigin;
@@ -51,3 +52,14 @@ impl Animatable for TransformOrigin {
         )
     }
 }
+
+impl ToAnimatedZero for TransformOrigin {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(Self::new(
+            self.horizontal.to_animated_zero()?,
+            self.vertical.to_animated_zero()?,
+            self.depth.to_animated_zero()?,
+        ))
+    }
+}

components/style/values/generics/text.rs

@@ -9,6 +9,7 @@ use cssparser::Parser;
 use parser::ParserContext;
 use properties::animated_properties::Animatable;
 use style_traits::ParseError;
+use values::animated::ToAnimatedZero;
 
 /// A generic value for the `initial-letter` property.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
@@ -93,6 +94,14 @@ impl<Value> Animatable for Spacing<Value>
     }
 }
 
+impl<V> ToAnimatedZero for Spacing<V>
+where
+    V: From<Au>,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+}
+
 /// A generic value for the `line-height` property.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToCss)]

ports/geckolib/glue.rs

@@ -119,6 +119,7 @@ use style::traversal::{ANIMATION_ONLY, DomTraversal, FOR_CSS_RULE_CHANGES, FOR_R
 use style::traversal::{TraversalDriver, TraversalFlags, UNSTYLED_CHILDREN_ONLY};
 use style::traversal::resolve_style;
 use style::values::{CustomIdent, KeyframesName};
+use style::values::animated::ToAnimatedZero;
 use style::values::computed::Context;
 use style_traits::{PARSING_MODE_DEFAULT, ToCss};
 use super::error_reporter::ErrorReporter;
@@ -377,7 +378,7 @@ pub extern "C" fn Servo_AnimationValues_GetZeroValue(
     -> RawServoAnimationValueStrong
 {
     let value_to_match = AnimationValue::as_arc(&value_to_match);
-    if let Ok(zero_value) = value_to_match.get_zero_value() {
+    if let Ok(zero_value) = value_to_match.to_animated_zero() {
         Arc::new(zero_value).into_strong()
     } else {
         RawServoAnimationValueStrong::null()

tests/unit/style/animated_properties.rs

@@ -4,7 +4,7 @@
 
 use app_units::Au;
 use cssparser::RGBA;
-use style::properties::animated_properties::{Animatable, IntermediateRGBA};
+use style::properties::animated_properties::Animatable;
 use style::properties::longhands::transform::computed_value::ComputedOperation as TransformOperation;
 use style::properties::longhands::transform::computed_value::T as TransformList;
 use style::values::animated::ToAnimatedValue;