diff --git a/components/style/properties/helpers/animated_properties.mako.rs b/components/style/properties/helpers/animated_properties.mako.rs
index 5c1b86f855f..c23aeec2a7c 100644
--- a/components/style/properties/helpers/animated_properties.mako.rs
+++ b/components/style/properties/helpers/animated_properties.mako.rs
@@ -641,6 +641,24 @@ impl Animatable for AnimationValue {
         }
     }
 
+    fn get_zero_value(&self) -> Option<Self> {
+        match self {
+            % for prop in data.longhands:
+                % if prop.animatable:
+                    % if prop.animation_value_type == "discrete":
+                        &AnimationValue::${prop.camel_case}(_) => {
+                            None
+                        }
+                    % else:
+                        &AnimationValue::${prop.camel_case}(ref base) => {
+                            base.get_zero_value().map(AnimationValue::${prop.camel_case})
+                        }
+                    % endif
+                % endif
+            % endfor
+        }
+    }
+
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (self, other) {
             % for prop in data.longhands:
@@ -697,6 +715,17 @@ 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.
+    fn get_zero_value(&self) -> Option<Self> {
+        None
+    }
+
     /// Compute distance between a value and another for a given property.
     fn compute_distance(&self, _other: &Self) -> Result<f64, ()>  { Err(()) }
 
@@ -746,6 +775,9 @@ 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) -> Option<Self> { Some(Au(0)) }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         self.0.compute_distance(&other.0)
@@ -795,6 +827,9 @@ impl Animatable for f32 {
         Ok((*self as f64 * self_portion + *other as f64 * other_portion) as f32)
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> { Some(0.) }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         Ok((*self - *other).abs() as f64)
@@ -808,6 +843,9 @@ impl Animatable for f64 {
         Ok(*self * self_portion + *other * other_portion)
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> { Some(0.) }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         Ok((*self - *other).abs())
@@ -821,6 +859,9 @@ impl Animatable for i32 {
         Ok((*self as f64 * self_portion + *other as f64 * other_portion).round() as i32)
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> { Some(0) }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         Ok((*self - *other).abs() as f64)
@@ -1007,6 +1048,9 @@ impl Animatable for LengthOrPercentage {
         }
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> { Some(LengthOrPercentage::zero()) }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (*self, *other) {
@@ -1079,6 +1123,18 @@ impl Animatable for LengthOrPercentageOrAuto {
         }
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> {
+        match *self {
+            LengthOrPercentageOrAuto::Length(_) |
+            LengthOrPercentageOrAuto::Percentage(_) |
+            LengthOrPercentageOrAuto::Calc(_) => {
+                Some(LengthOrPercentageOrAuto::Length(Au(0)))
+            },
+            LengthOrPercentageOrAuto::Auto => { None },
+        }
+    }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (*self, *other) {
@@ -1149,6 +1205,18 @@ impl Animatable for LengthOrPercentageOrNone {
         }
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> {
+        match *self {
+            LengthOrPercentageOrNone::Length(_) |
+            LengthOrPercentageOrNone::Percentage(_) |
+            LengthOrPercentageOrNone::Calc(_) => {
+                Some(LengthOrPercentageOrNone::Length(Au(0)))
+            },
+            LengthOrPercentageOrNone::None => { None },
+        }
+    }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (*self, *other) {
@@ -1223,7 +1291,8 @@ impl Animatable for FontWeight {
     fn add_weighted(&self, other: &Self, self_portion: f64, other_portion: f64) -> Result<Self, ()> {
         let a = (*self as u32) as f64;
         let b = (*other as u32) as f64;
-        let weight = a * self_portion + b * other_portion;
+        const NORMAL: f64 = 400.;
+        let weight = (a - NORMAL) * self_portion + (b - NORMAL) * other_portion + NORMAL;
         Ok(if weight < 150. {
             FontWeight::Weight100
         } else if weight < 250. {
@@ -1245,6 +1314,9 @@ impl Animatable for FontWeight {
         })
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> { Some(FontWeight::Weight400) }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         let a = (*self as u32) as f64;
@@ -1306,6 +1378,11 @@ impl Into<FontStretch> for f64 {
     }
 }
 
+// Like std::macros::try!, but for Option<>.
+macro_rules! option_try {
+    ($e:expr) => (match $e { Some(e) => e, None => return None })
+}
+
 /// https://drafts.csswg.org/css-transitions/#animtype-simple-list
 impl<H: Animatable, V: Animatable> Animatable for generic_position::Position<H, V> {
     #[inline]
@@ -1318,6 +1395,14 @@ impl<H: Animatable, V: Animatable> Animatable for generic_position::Position<H,
         })
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> {
+        Some(generic_position::Position {
+            horizontal: option_try!(self.horizontal.get_zero_value()),
+            vertical: option_try!(self.vertical.get_zero_value()),
+        })
+    }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         self.compute_squared_distance(other).map(|sd| sd.sqrt())
@@ -2493,6 +2578,9 @@ impl Animatable for TransformList {
             }
         }
     }
+
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> { Some(TransformList(None)) }
 }
 
 impl<T, U> Animatable for Either<T, U>
@@ -2514,6 +2602,14 @@ impl<T, U> Animatable for Either<T, U>
         }
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> {
+        match *self {
+            Either::First(ref this) => { this.get_zero_value().map(Either::First) },
+            Either::Second(ref this) => { this.get_zero_value().map(Either::Second) },
+        }
+    }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         match (self, other) {
@@ -2616,6 +2712,11 @@ impl Animatable for IntermediateRGBA {
         }
     }
 
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> {
+        Some(IntermediateRGBA::new(0., 0., 0., 1.))
+    }
+
     #[inline]
     fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
         self.compute_squared_distance(other).map(|sq| sq.sqrt())