Auto merge of #18239 - servo:derive-all-the-things, r=emilio

Refactor how we handle trait bounds in style_derive

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27 files changed, 649 insertions, 844 deletions

diff --git a/components/style/properties/helpers/animated_properties.mako.rs b/components/style/properties/helpers/animated_properties.mako.rs
index e3c831c8bc9..4c2057f61d9 100644
--- a/components/style/properties/helpers/animated_properties.mako.rs
+++ b/components/style/properties/helpers/animated_properties.mako.rs
@@ -48,9 +48,10 @@ use values::animated::effects::TextShadowList as AnimatedTextShadowList;
 use values::computed::{Angle, BorderCornerRadius, CalcLengthOrPercentage};
 use values::computed::{ClipRect, Context, ComputedUrl, ComputedValueAsSpecified};
 use values::computed::{LengthOrPercentage, LengthOrPercentageOrAuto};
-use values::computed::{LengthOrPercentageOrNone, MaxLength, MozLength, NonNegativeAu};
+use values::computed::{LengthOrPercentageOrNone, MaxLength, NonNegativeAu};
 use values::computed::{NonNegativeNumber, Number, NumberOrPercentage, Percentage};
 use values::computed::{PositiveIntegerOrAuto, ToComputedValue};
+#[cfg(feature = "gecko")] use values::computed::MozLength;
 use values::computed::length::{NonNegativeLengthOrAuto, NonNegativeLengthOrNormal};
 use values::computed::length::NonNegativeLengthOrPercentage;
 use values::distance::{ComputeSquaredDistance, SquaredDistance};
@@ -811,73 +812,6 @@ impl Animate for CalcLengthOrPercentage {
     }
 }
 
-/// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
-impl Animate for LengthOrPercentage {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (
-                &LengthOrPercentage::Length(ref this),
-                &LengthOrPercentage::Length(ref other),
-            ) => {
-                Ok(LengthOrPercentage::Length(this.animate(other, procedure)?))
-            },
-            (
-                &LengthOrPercentage::Percentage(ref this),
-                &LengthOrPercentage::Percentage(ref other),
-            ) => {
-                Ok(LengthOrPercentage::Percentage(this.animate(other, procedure)?))
-            },
-            (this, other) => {
-                // Special handling for zero values since these should not require calc().
-                if this.is_definitely_zero() {
-                    return other.to_animated_zero()?.animate(other, procedure);
-                }
-                if other.is_definitely_zero() {
-                    return this.animate(&this.to_animated_zero()?, procedure);
-                }
-
-                let this = CalcLengthOrPercentage::from(*this);
-                let other = CalcLengthOrPercentage::from(*other);
-                Ok(LengthOrPercentage::Calc(this.animate(&other, procedure)?))
-            }
-        }
-    }
-}
-
-/// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
-impl Animate for LengthOrPercentageOrAuto {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (
-                &LengthOrPercentageOrAuto::Length(ref this),
-                &LengthOrPercentageOrAuto::Length(ref other),
-            ) => {
-                Ok(LengthOrPercentageOrAuto::Length(this.animate(other, procedure)?))
-            },
-            (
-                &LengthOrPercentageOrAuto::Percentage(ref this),
-                &LengthOrPercentageOrAuto::Percentage(ref other),
-            ) => {
-                Ok(LengthOrPercentageOrAuto::Percentage(
-                    this.animate(other, procedure)?,
-                ))
-            },
-            (&LengthOrPercentageOrAuto::Auto, &LengthOrPercentageOrAuto::Auto) => {
-                Ok(LengthOrPercentageOrAuto::Auto)
-            },
-            (this, other) => {
-                let this: Option<CalcLengthOrPercentage> = From::from(*this);
-                let other: Option<CalcLengthOrPercentage> = From::from(*other);
-                Ok(LengthOrPercentageOrAuto::Calc(
-                    this.animate(&other, procedure)?.ok_or(())?,
-                ))
-            },
-        }
-    }
-}
-
 impl ToAnimatedZero for LengthOrPercentageOrAuto {
     #[inline]
     fn to_animated_zero(&self) -> Result<Self, ()> {
@@ -892,39 +826,6 @@ impl ToAnimatedZero for LengthOrPercentageOrAuto {
     }
 }
 
-/// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
-impl Animate for LengthOrPercentageOrNone {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (
-                &LengthOrPercentageOrNone::Length(ref this),
-                &LengthOrPercentageOrNone::Length(ref other),
-            ) => {
-                Ok(LengthOrPercentageOrNone::Length(this.animate(other, procedure)?))
-            },
-            (
-                &LengthOrPercentageOrNone::Percentage(ref this),
-                &LengthOrPercentageOrNone::Percentage(ref other),
-            ) => {
-                Ok(LengthOrPercentageOrNone::Percentage(
-                    this.animate(other, procedure)?,
-                ))
-            }
-            (&LengthOrPercentageOrNone::None, &LengthOrPercentageOrNone::None) => {
-                Ok(LengthOrPercentageOrNone::None)
-            },
-            (this, other) => {
-                let this = <Option<CalcLengthOrPercentage>>::from(*this);
-                let other = <Option<CalcLengthOrPercentage>>::from(*other);
-                Ok(LengthOrPercentageOrNone::Calc(
-                    this.animate(&other, procedure)?.ok_or(())?,
-                ))
-            },
-        }
-    }
-}
-
 impl ToAnimatedZero for LengthOrPercentageOrNone {
     #[inline]
     fn to_animated_zero(&self) -> Result<Self, ()> {
@@ -939,54 +840,6 @@ impl ToAnimatedZero for LengthOrPercentageOrNone {
     }
 }
 
-/// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
-impl Animate for MozLength {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (
-                &MozLength::LengthOrPercentageOrAuto(ref this),
-                &MozLength::LengthOrPercentageOrAuto(ref other),
-            ) => {
-                Ok(MozLength::LengthOrPercentageOrAuto(
-                    this.animate(other, procedure)?,
-                ))
-            }
-            _ => Err(()),
-        }
-    }
-}
-
-impl ToAnimatedZero for MozLength {
-    #[inline]
-    fn to_animated_zero(&self) -> Result<Self, ()> {
-        match *self {
-            MozLength::LengthOrPercentageOrAuto(ref length) => {
-                Ok(MozLength::LengthOrPercentageOrAuto(length.to_animated_zero()?))
-            },
-            _ => Err(())
-        }
-    }
-}
-
-/// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
-impl Animate for MaxLength {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (
-                &MaxLength::LengthOrPercentageOrNone(ref this),
-                &MaxLength::LengthOrPercentageOrNone(ref other),
-            ) => {
-                Ok(MaxLength::LengthOrPercentageOrNone(
-                    this.animate(other, procedure)?,
-                ))
-            },
-            _ => Err(()),
-        }
-    }
-}
-
 impl ToAnimatedZero for MaxLength {
     #[inline]
     fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
@@ -2431,27 +2284,6 @@ pub type IntermediateSVGPaint = SVGPaint<AnimatedRGBA, ComputedUrl>;
 /// Animated SVGPaintKind
 pub type IntermediateSVGPaintKind = SVGPaintKind<AnimatedRGBA, ComputedUrl>;
 
-impl Animate for IntermediateSVGPaint {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        Ok(IntermediateSVGPaint {
-            kind: self.kind.animate(&other.kind, procedure)?,
-            fallback: self.fallback.animate(&other.fallback, procedure)?,
-        })
-    }
-}
-
-impl ComputeSquaredDistance for IntermediateSVGPaint {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        // FIXME(nox): This should be derived.
-        Ok(
-            self.kind.compute_squared_distance(&other.kind)? +
-            self.fallback.compute_squared_distance(&other.fallback)?,
-        )
-    }
-}
-
 impl ToAnimatedZero for IntermediateSVGPaint {
     #[inline]
     fn to_animated_zero(&self) -> Result<Self, ()> {
@@ -2462,56 +2294,6 @@ impl ToAnimatedZero for IntermediateSVGPaint {
     }
 }
 
-impl Animate for IntermediateSVGPaintKind {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (&SVGPaintKind::Color(ref this), &SVGPaintKind::Color(ref other)) => {
-                Ok(SVGPaintKind::Color(this.animate(other, procedure)?))
-            },
-            (&SVGPaintKind::ContextFill, &SVGPaintKind::ContextFill) => Ok(SVGPaintKind::ContextFill),
-            (&SVGPaintKind::ContextStroke, &SVGPaintKind::ContextStroke) => Ok(SVGPaintKind::ContextStroke),
-            _ => {
-                // FIXME: Context values should be interpolable with colors,
-                // Gecko doesn't implement this behavior either.
-                Err(())
-            }
-        }
-    }
-}
-
-impl ComputeSquaredDistance for IntermediateSVGPaintKind {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        match (self, other) {
-            (&SVGPaintKind::Color(ref this), &SVGPaintKind::Color(ref other)) => {
-                this.compute_squared_distance(other)
-            }
-            (&SVGPaintKind::None, &SVGPaintKind::None) |
-            (&SVGPaintKind::ContextFill, &SVGPaintKind::ContextFill) |
-            (&SVGPaintKind::ContextStroke, &SVGPaintKind::ContextStroke) => {
-                Ok(SquaredDistance::Value(0.))
-            },
-            _ => Err(())
-        }
-    }
-}
-
-impl ToAnimatedZero for IntermediateSVGPaintKind {
-    #[inline]
-    fn to_animated_zero(&self) -> Result<Self, ()> {
-        match *self {
-            SVGPaintKind::Color(ref color) => {
-                Ok(SVGPaintKind::Color(color.to_animated_zero()?))
-            },
-            SVGPaintKind::None |
-            SVGPaintKind::ContextFill |
-            SVGPaintKind::ContextStroke => Ok(self.clone()),
-            _ => Err(()),
-        }
-    }
-}
-
 impl From<NonNegativeLengthOrPercentage> for NumberOrPercentage {
     fn from(lop: NonNegativeLengthOrPercentage) -> NumberOrPercentage {
         lop.0.into()
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index e4d257d02a6..7dc51dca3d4 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -1116,12 +1116,12 @@ ${helpers.single_keyword_system("font-variant-caps",
 
     pub mod computed_value {
         use values::CSSFloat;
-        use values::animated::{Animate, Procedure, ToAnimatedValue, ToAnimatedZero};
-        use values::distance::{ComputeSquaredDistance, SquaredDistance};
+        use values::animated::{ToAnimatedValue, ToAnimatedZero};
 
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-        #[derive(Clone, Copy, Debug, PartialEq, ToCss)]
+        #[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug, PartialEq, ToCss)]
         pub enum T {
+            #[animation(error)]
             None,
             Number(CSSFloat),
         }
@@ -1136,27 +1136,6 @@ ${helpers.single_keyword_system("font-variant-caps",
             }
         }
 
-        impl Animate for T {
-            fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-                match (self, other) {
-                    (&T::Number(ref number), &T::Number(ref other)) => {
-                        Ok(T::Number(number.animate(other, procedure)?))
-                    },
-                    _ => Err(()),
-                }
-            }
-        }
-
-        impl ComputeSquaredDistance for T {
-            #[inline]
-            fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-                match (self, other) {
-                    (&T::Number(ref this), &T::Number(ref other)) => this.compute_squared_distance(other),
-                    _ => Err(()),
-                }
-            }
-        }
-
         impl ToAnimatedZero for T {
             #[inline]
             fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
@@ -2265,11 +2244,8 @@ https://drafts.csswg.org/css-fonts-4/#low-level-font-variation-settings-control-
                    predefined_type="Number" gecko_ffi_name="mScriptSizeMultiplier"
                    spec="Internal (not web-exposed)"
                    internal="True">
-    use values::computed::ComputedValueAsSpecified;
     pub use self::computed_value::T as SpecifiedValue;
 
-    impl ComputedValueAsSpecified for SpecifiedValue {}
-
     pub mod computed_value {
         pub type T = f32;
     }
diff --git a/components/style/values/animated/effects.rs b/components/style/values/animated/effects.rs
index cdde2894549..eb03bd9565b 100644
--- a/components/style/values/animated/effects.rs
+++ b/components/style/values/animated/effects.rs
@@ -139,21 +139,6 @@ impl ToAnimatedValue for ComputedTextShadowList {
     }
 }
 
-// FIXME(nox): This could be derived if we implement Animate for bool.
-impl Animate for BoxShadow {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        if self.inset != other.inset {
-            return Err(());
-        }
-        Ok(BoxShadow {
-            base: self.base.animate(&other.base, procedure)?,
-            spread: self.spread.animate(&other.spread, procedure)?,
-            inset: self.inset,
-        })
-    }
-}
-
 impl ComputeSquaredDistance for BoxShadow {
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
@@ -167,17 +152,6 @@ impl ComputeSquaredDistance 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;
 
diff --git a/components/style/values/animated/mod.rs b/components/style/values/animated/mod.rs
index 71f6e2bd4db..c2d42192fdd 100644
--- a/components/style/values/animated/mod.rs
+++ b/components/style/values/animated/mod.rs
@@ -28,7 +28,19 @@ use values::specified::url::SpecifiedUrl;
 pub mod color;
 pub mod effects;
 
-/// Animating from one value to another.
+/// Animate from one value to another.
+///
+/// This trait is derivable with `#[derive(Animate)]`. The derived
+/// implementation uses a `match` expression with identical patterns for both
+/// `self` and `other`, calling `Animate::animate` on each fields of the values.
+/// If a field is annotated with `#[animation(constant)]`, the two values should
+/// be equal or an error is returned.
+///
+/// If a variant is annotated with `#[animation(error)]`, the corresponding
+/// `match` arm is not generated.
+///
+/// If the two values are not similar, an error is returned unless a fallback
+/// function has been specified through `#[animate(fallback)]`.
 pub trait Animate: Sized {
     /// Animate a value towards another one, given an animation procedure.
     fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()>;
@@ -51,6 +63,8 @@ pub enum Procedure {
 /// Conversion between computed values and intermediate values for animations.
 ///
 /// Notably, colors are represented as four floats during animations.
+///
+/// This trait is derivable with `#[derive(ToAnimatedValue)]`.
 pub trait ToAnimatedValue {
     /// The type of the animated value.
     type AnimatedValue;
@@ -66,6 +80,13 @@ pub trait ToAnimatedValue {
 pub trait AnimatedValueAsComputed {}
 
 /// Returns a value similar to `self` that represents zero.
+///
+/// This trait is derivable with `#[derive(ToAnimatedValue)]`. If a field is
+/// annotated with `#[animation(constant)]`, a clone of its value will be used
+/// instead of calling `ToAnimatedZero::to_animated_zero` on it.
+///
+/// If a variant is annotated with `#[animation(error)]`, the corresponding
+/// `match` arm is not generated.
 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
diff --git a/components/style/values/computed/angle.rs b/components/style/values/computed/angle.rs
index d5461d1cbf9..467a9501196 100644
--- a/components/style/values/computed/angle.rs
+++ b/components/style/values/computed/angle.rs
@@ -12,8 +12,10 @@ use values::animated::{Animate, Procedure};
 use values::distance::{ComputeSquaredDistance, SquaredDistance};
 
 /// A computed angle.
+#[animate(fallback = "Self::animate_fallback")]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf, Deserialize, Serialize))]
-#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, PartialOrd, ToAnimatedZero)]
+#[derive(Animate, Clone, Copy, Debug, HasViewportPercentage, PartialEq)]
+#[derive(PartialOrd, ToAnimatedZero)]
 pub enum Angle {
     /// An angle with degree unit.
     Degree(CSSFloat),
@@ -62,26 +64,11 @@ impl Angle {
     pub fn zero() -> Self {
         Angle::Radian(0.0)
     }
-}
 
-/// https://drafts.csswg.org/css-transitions/#animtype-number
-impl Animate for Angle {
+    /// https://drafts.csswg.org/css-transitions/#animtype-number
     #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (&Angle::Degree(ref this), &Angle::Degree(ref other)) => {
-                Ok(Angle::Degree(this.animate(other, procedure)?))
-            },
-            (&Angle::Gradian(ref this), &Angle::Gradian(ref other)) => {
-                Ok(Angle::Gradian(this.animate(other, procedure)?))
-            },
-            (&Angle::Turn(ref this), &Angle::Turn(ref other)) => {
-                Ok(Angle::Turn(this.animate(other, procedure)?))
-            },
-            _ => {
-                Ok(Angle::from_radians(self.radians().animate(&other.radians(), procedure)?))
-            },
-        }
+    fn animate_fallback(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        Ok(Angle::from_radians(self.radians().animate(&other.radians(), procedure)?))
     }
 }
 
diff --git a/components/style/values/computed/background.rs b/components/style/values/computed/background.rs
index 82b6ba74f28..f3180c54ebe 100644
--- a/components/style/values/computed/background.rs
+++ b/components/style/values/computed/background.rs
@@ -6,7 +6,7 @@
 
 use properties::animated_properties::RepeatableListAnimatable;
 use properties::longhands::background_size::computed_value::T as BackgroundSizeList;
-use values::animated::{Animate, Procedure, ToAnimatedValue, ToAnimatedZero};
+use values::animated::{ToAnimatedValue, ToAnimatedZero};
 use values::computed::length::LengthOrPercentageOrAuto;
 use values::generics::background::BackgroundSize as GenericBackgroundSize;
 
@@ -15,23 +15,6 @@ pub type BackgroundSize = GenericBackgroundSize<LengthOrPercentageOrAuto>;
 
 impl RepeatableListAnimatable for BackgroundSize {}
 
-impl Animate for BackgroundSize {
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (
-                &GenericBackgroundSize::Explicit { width: self_width, height: self_height },
-                &GenericBackgroundSize::Explicit { width: other_width, height: other_height },
-            ) => {
-                Ok(GenericBackgroundSize::Explicit {
-                    width: self_width.animate(&other_width, procedure)?,
-                    height: self_height.animate(&other_height, procedure)?,
-                })
-            }
-            _ => Err(()),
-        }
-    }
-}
-
 impl ToAnimatedZero for BackgroundSize {
     #[inline]
     fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
diff --git a/components/style/values/computed/image.rs b/components/style/values/computed/image.rs
index a4a51b7bae6..952e213ccb9 100644
--- a/components/style/values/computed/image.rs
+++ b/components/style/values/computed/image.rs
@@ -18,8 +18,7 @@ use values::generics::image::{CompatMode, ColorStop as GenericColorStop, EndingS
 use values::generics::image::{Gradient as GenericGradient, GradientItem as GenericGradientItem};
 use values::generics::image::{Image as GenericImage, GradientKind as GenericGradientKind};
 use values::generics::image::{LineDirection as GenericLineDirection, MozImageRect as GenericMozImageRect};
-use values::specified::image::{Gradient as SpecifiedGradient, LineDirection as SpecifiedLineDirection};
-use values::specified::image::{GradientKind as SpecifiedGradientKind};
+use values::specified::image::LineDirection as SpecifiedLineDirection;
 use values::specified::position::{X, Y};
 
 /// A computed image layer.
@@ -181,55 +180,3 @@ impl ToComputedValue for SpecifiedLineDirection {
         }
     }
 }
-
-impl ToComputedValue for SpecifiedGradient {
-    type ComputedValue = Gradient;
-
-    fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
-        Self::ComputedValue {
-            kind: self.kind.to_computed_value(context),
-            items: self.items.to_computed_value(context),
-            repeating: self.repeating,
-            compat_mode: self.compat_mode
-        }
-    }
-
-    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
-        Self {
-            kind: SpecifiedGradientKind::from_computed_value(&computed.kind),
-            items: ToComputedValue::from_computed_value(&computed.items),
-            repeating: computed.repeating,
-            compat_mode: computed.compat_mode
-        }
-    }
-}
-
-impl ToComputedValue for SpecifiedGradientKind {
-    type ComputedValue = GradientKind;
-
-    fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
-        match self {
-            &GenericGradientKind::Linear(ref line_direction) => {
-                GenericGradientKind::Linear(line_direction.to_computed_value(context))
-            },
-            &GenericGradientKind::Radial(ref ending_shape, ref position, ref angle) => {
-                GenericGradientKind::Radial(ending_shape.to_computed_value(context),
-                                            position.to_computed_value(context),
-                                            angle.map(|angle| angle.to_computed_value(context)))
-            }
-        }
-    }
-
-    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
-        match *computed {
-            GenericGradientKind::Linear(line_direction) => {
-                GenericGradientKind::Linear(SpecifiedLineDirection::from_computed_value(&line_direction))
-            },
-            GenericGradientKind::Radial(ending_shape, position, angle) => {
-                GenericGradientKind::Radial(ToComputedValue::from_computed_value(&ending_shape),
-                                            ToComputedValue::from_computed_value(&position),
-                                            angle.map(|angle| ToComputedValue::from_computed_value(&angle)))
-            }
-        }
-    }
-}
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index b4f4cfc5101..d29f5aa87fc 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -11,7 +11,7 @@ use style_traits::ToCss;
 use style_traits::values::specified::AllowedLengthType;
 use super::{Number, ToComputedValue, Context, Percentage};
 use values::{Auto, CSSFloat, Either, ExtremumLength, None_, Normal, specified};
-use values::animated::ToAnimatedZero;
+use values::animated::{Animate, Procedure, ToAnimatedZero};
 use values::computed::{NonNegativeAu, NonNegativeNumber};
 use values::distance::{ComputeSquaredDistance, SquaredDistance};
 use values::generics::NonNegative;
@@ -64,26 +64,16 @@ impl ToComputedValue for specified::Length {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, PartialEq, ToAnimatedZero)]
 pub struct CalcLengthOrPercentage {
+    #[animation(constant)]
     pub clamping_mode: AllowedLengthType,
     length: Au,
     pub percentage: Option<Percentage>,
 }
 
-impl ToAnimatedZero for CalcLengthOrPercentage {
-    #[inline]
-    fn to_animated_zero(&self) -> Result<Self, ()> {
-        Ok(CalcLengthOrPercentage {
-            clamping_mode: self.clamping_mode,
-            length: self.length.to_animated_zero()?,
-            percentage: self.percentage.to_animated_zero()?,
-        })
-    }
-}
-
 impl ComputeSquaredDistance for CalcLengthOrPercentage {
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
@@ -285,28 +275,47 @@ impl ToComputedValue for specified::CalcLengthOrPercentage {
 }
 
 #[allow(missing_docs)]
+#[animate(fallback = "Self::animate_fallback")]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, PartialEq, ToAnimatedZero, ToCss)]
+#[css(derive_debug)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, PartialEq)]
+#[derive(ToAnimatedZero, ToCss)]
+#[distance(fallback = "Self::compute_squared_distance_fallback")]
 pub enum LengthOrPercentage {
     Length(Au),
     Percentage(Percentage),
     Calc(CalcLengthOrPercentage),
 }
 
-impl ComputeSquaredDistance for LengthOrPercentage {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        match (self, other) {
-            (&LengthOrPercentage::Length(ref this), &LengthOrPercentage::Length(ref other)) => {
-                this.compute_squared_distance(other)
-            },
-            (&LengthOrPercentage::Percentage(ref this), &LengthOrPercentage::Percentage(ref other)) => {
-                this.compute_squared_distance(other)
-            },
-            (this, other) => {
-                CalcLengthOrPercentage::compute_squared_distance(&(*this).into(), &(*other).into())
-            }
+impl LengthOrPercentage {
+    /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
+    fn animate_fallback(
+        &self,
+        other: &Self,
+        procedure: Procedure,
+    ) -> Result<Self, ()> {
+        // Special handling for zero values since these should not require calc().
+        if self.is_definitely_zero() {
+            return other.to_animated_zero()?.animate(other, procedure);
         }
+        if other.is_definitely_zero() {
+            return self.animate(&self.to_animated_zero()?, procedure);
+        }
+
+        let this = CalcLengthOrPercentage::from(*self);
+        let other = CalcLengthOrPercentage::from(*other);
+        Ok(LengthOrPercentage::Calc(this.animate(&other, procedure)?))
+    }
+
+    #[inline]
+    fn compute_squared_distance_fallback(
+        &self,
+        other: &Self,
+    ) -> Result<SquaredDistance, ()> {
+        CalcLengthOrPercentage::compute_squared_distance(
+            &(*self).into(),
+            &(*other).into(),
+        )
     }
 }
 
@@ -379,16 +388,6 @@ impl LengthOrPercentage {
     }
 }
 
-impl fmt::Debug for LengthOrPercentage {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        match *self {
-            LengthOrPercentage::Length(length) => write!(f, "{:?}", length),
-            LengthOrPercentage::Percentage(percentage) => write!(f, "{}%", percentage.0 * 100.),
-            LengthOrPercentage::Calc(calc) => write!(f, "{:?}", calc),
-        }
-    }
-}
-
 impl ToComputedValue for specified::LengthOrPercentage {
     type ComputedValue = LengthOrPercentage;
 
@@ -426,8 +425,11 @@ impl ToComputedValue for specified::LengthOrPercentage {
 }
 
 #[allow(missing_docs)]
+#[animate(fallback = "Self::animate_fallback")]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, PartialEq, ToCss)]
+#[css(derive_debug)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, PartialEq, ToCss)]
+#[distance(fallback = "Self::compute_squared_distance_fallback")]
 pub enum LengthOrPercentageOrAuto {
     Length(Au),
     Percentage(Percentage),
@@ -435,20 +437,29 @@ pub enum LengthOrPercentageOrAuto {
     Calc(CalcLengthOrPercentage),
 }
 
-impl ComputeSquaredDistance for LengthOrPercentageOrAuto {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        match (self, other) {
-            (&LengthOrPercentageOrAuto::Length(ref this), &LengthOrPercentageOrAuto::Length(ref other)) => {
-                this.compute_squared_distance(other)
-            },
-            (&LengthOrPercentageOrAuto::Percentage(ref this), &LengthOrPercentageOrAuto::Percentage(ref other)) => {
-                this.compute_squared_distance(other)
-            },
-            (this, other) => {
-                <Option<CalcLengthOrPercentage>>::compute_squared_distance(&(*this).into(), &(*other).into())
-            }
-        }
+impl LengthOrPercentageOrAuto {
+    /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
+    fn animate_fallback(
+        &self,
+        other: &Self,
+        procedure: Procedure,
+    ) -> Result<Self, ()> {
+        let this = <Option<CalcLengthOrPercentage>>::from(*self);
+        let other = <Option<CalcLengthOrPercentage>>::from(*other);
+        Ok(LengthOrPercentageOrAuto::Calc(
+            this.animate(&other, procedure)?.ok_or(())?,
+        ))
+    }
+
+    #[inline]
+    fn compute_squared_distance_fallback(
+        &self,
+        other: &Self,
+    ) -> Result<SquaredDistance, ()> {
+        <Option<CalcLengthOrPercentage>>::compute_squared_distance(
+            &(*self).into(),
+            &(*other).into(),
+        )
     }
 }
 
@@ -467,17 +478,6 @@ impl LengthOrPercentageOrAuto {
     }
 }
 
-impl fmt::Debug for LengthOrPercentageOrAuto {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        match *self {
-            LengthOrPercentageOrAuto::Length(length) => write!(f, "{:?}", length),
-            LengthOrPercentageOrAuto::Percentage(percentage) => write!(f, "{}%", percentage.0 * 100.),
-            LengthOrPercentageOrAuto::Auto => write!(f, "auto"),
-            LengthOrPercentageOrAuto::Calc(calc) => write!(f, "{:?}", calc),
-        }
-    }
-}
-
 impl ToComputedValue for specified::LengthOrPercentageOrAuto {
     type ComputedValue = LengthOrPercentageOrAuto;
 
@@ -521,8 +521,11 @@ impl ToComputedValue for specified::LengthOrPercentageOrAuto {
 }
 
 #[allow(missing_docs)]
+#[animate(fallback = "Self::animate_fallback")]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, PartialEq, ToCss)]
+#[css(derive_debug)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, PartialEq, ToCss)]
+#[distance(fallback = "Self::compute_squared_distance_fallback")]
 pub enum LengthOrPercentageOrNone {
     Length(Au),
     Percentage(Percentage),
@@ -530,20 +533,28 @@ pub enum LengthOrPercentageOrNone {
     None,
 }
 
-impl ComputeSquaredDistance for LengthOrPercentageOrNone {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        match (self, other) {
-            (&LengthOrPercentageOrNone::Length(ref this), &LengthOrPercentageOrNone::Length(ref other)) => {
-                this.compute_squared_distance(other)
-            },
-            (&LengthOrPercentageOrNone::Percentage(ref this), &LengthOrPercentageOrNone::Percentage(ref other)) => {
-                this.compute_squared_distance(other)
-            },
-            (this, other) => {
-                <Option<CalcLengthOrPercentage>>::compute_squared_distance(&(*this).into(), &(*other).into())
-            }
-        }
+impl LengthOrPercentageOrNone {
+    /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
+    fn animate_fallback(
+        &self,
+        other: &Self,
+        procedure: Procedure,
+    ) -> Result<Self, ()> {
+        let this = <Option<CalcLengthOrPercentage>>::from(*self);
+        let other = <Option<CalcLengthOrPercentage>>::from(*other);
+        Ok(LengthOrPercentageOrNone::Calc(
+            this.animate(&other, procedure)?.ok_or(())?,
+        ))
+    }
+
+    fn compute_squared_distance_fallback(
+        &self,
+        other: &Self,
+    ) -> Result<SquaredDistance, ()> {
+        <Option<CalcLengthOrPercentage>>::compute_squared_distance(
+            &(*self).into(),
+            &(*other).into(),
+        )
     }
 }
 
@@ -559,17 +570,6 @@ impl LengthOrPercentageOrNone {
     }
 }
 
-impl fmt::Debug for LengthOrPercentageOrNone {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        match *self {
-            LengthOrPercentageOrNone::Length(length) => write!(f, "{:?}", length),
-            LengthOrPercentageOrNone::Percentage(percentage) => write!(f, "{}%", percentage.0 * 100.),
-            LengthOrPercentageOrNone::Calc(calc) => write!(f, "{:?}", calc),
-            LengthOrPercentageOrNone::None => write!(f, "none"),
-        }
-    }
-}
-
 impl ToComputedValue for specified::LengthOrPercentageOrNone {
     type ComputedValue = LengthOrPercentageOrNone;
 
@@ -695,28 +695,14 @@ pub type NonNegativeLengthOrNumber = Either<NonNegativeLength, NonNegativeNumber
 /// See values/specified/length.rs for more details.
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug, PartialEq)]
+#[derive(ToAnimatedZero, ToCss)]
 pub enum MozLength {
     LengthOrPercentageOrAuto(LengthOrPercentageOrAuto),
+    #[animation(error)]
     ExtremumLength(ExtremumLength),
 }
 
-impl ComputeSquaredDistance for MozLength {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        match (self, other) {
-            (&MozLength::LengthOrPercentageOrAuto(ref this), &MozLength::LengthOrPercentageOrAuto(ref other)) => {
-                this.compute_squared_distance(other)
-            },
-            _ => {
-                // FIXME(nox): Should this return `Ok(SquaredDistance::Value(1.))`
-                // when `self` and `other` are the same extremum value?
-                Err(())
-            },
-        }
-    }
-}
-
 impl MozLength {
     /// Returns the `auto` value.
     pub fn auto() -> Self {
@@ -755,28 +741,13 @@ impl ToComputedValue for specified::MozLength {
 /// See values/specified/length.rs for more details.
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug, PartialEq, ToCss)]
 pub enum MaxLength {
     LengthOrPercentageOrNone(LengthOrPercentageOrNone),
+    #[animation(error)]
     ExtremumLength(ExtremumLength),
 }
 
-impl ComputeSquaredDistance for MaxLength {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        match (self, other) {
-            (&MaxLength::LengthOrPercentageOrNone(ref this), &MaxLength::LengthOrPercentageOrNone(ref other)) => {
-                this.compute_squared_distance(other)
-            },
-            _ => {
-                // FIXME(nox): Should this return `Ok(SquaredDistance::Value(1.))`
-                // when `self` and `other` are the same extremum value?
-                Err(())
-            },
-        }
-    }
-}
-
 impl MaxLength {
     /// Returns the `none` value.
     pub fn none() -> Self {
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index 8a15ef1162c..86aaf29b1c5 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -199,6 +199,11 @@ impl<'a, 'cx, 'cx_a: 'cx, S: ToComputedValue + 'a> Iterator for ComputedVecIter<
 }
 
 /// A trait to represent the conversion between computed and specified values.
+///
+/// This trait is derivable with `#[derive(ToComputedValue)]`. The derived
+/// implementation just calls `ToComputedValue::to_computed_value` on each field
+/// of the passed value, or `Clone::clone` if the field is annotated with
+/// `#[compute(clone)]`.
 pub trait ToComputedValue {
     /// The computed value type we're going to be converted to.
     type ComputedValue;
@@ -327,6 +332,7 @@ impl<T> ToComputedValue for T
 
 impl ComputedValueAsSpecified for Atom {}
 impl ComputedValueAsSpecified for bool {}
+impl ComputedValueAsSpecified for f32 {}
 
 impl ComputedValueAsSpecified for specified::BorderStyle {}
 
diff --git a/components/style/values/distance.rs b/components/style/values/distance.rs
index d4eb91f112f..351f1eb021f 100644
--- a/components/style/values/distance.rs
+++ b/components/style/values/distance.rs
@@ -10,6 +10,17 @@ use std::iter::Sum;
 use std::ops::Add;
 
 /// A trait to compute squared distances between two animatable values.
+///
+/// This trait is derivable with `#[derive(ComputeSquaredDistance)]`. The derived
+/// implementation uses a `match` expression with identical patterns for both
+/// `self` and `other`, calling `ComputeSquaredDistance::compute_squared_distance`
+/// on each fields of the values.
+///
+/// If a variant is annotated with `#[animation(error)]`, the corresponding
+/// `match` arm is not generated.
+///
+/// If the two values are not similar, an error is returned unless a fallback
+/// function has been specified through `#[distance(fallback)]`.
 pub trait ComputeSquaredDistance {
     /// Computes the squared distance between two animatable values.
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()>;
diff --git a/components/style/values/generics/background.rs b/components/style/values/generics/background.rs
index 8a79691f3b9..abcfb655eec 100644
--- a/components/style/values/generics/background.rs
+++ b/components/style/values/generics/background.rs
@@ -5,8 +5,9 @@
 //! Generic types for CSS values related to backgrounds.
 
 /// A generic value for the `background-size` property.
-#[derive(Clone, ComputeSquaredDistance, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue, ToCss)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug)]
+#[derive(HasViewportPercentage, PartialEq, ToComputedValue, ToCss)]
 pub enum BackgroundSize<LengthOrPercentageOrAuto> {
     /// `<width> <height>`
     Explicit {
@@ -16,8 +17,10 @@ pub enum BackgroundSize<LengthOrPercentageOrAuto> {
         height: LengthOrPercentageOrAuto
     },
     /// `cover`
+    #[animation(error)]
     Cover,
     /// `contain`
+    #[animation(error)]
     Contain,
 }
 
diff --git a/components/style/values/generics/basic_shape.rs b/components/style/values/generics/basic_shape.rs
index 99de026d706..019ed6599e2 100644
--- a/components/style/values/generics/basic_shape.rs
+++ b/components/style/values/generics/basic_shape.rs
@@ -44,11 +44,18 @@ add_impls_for_keyword_enum!(ShapeBox);
 /// A shape source, for some reference box.
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToComputedValue, ToCss)]
+#[derive(Animate, Clone, Debug, PartialEq, ToComputedValue, ToCss)]
 pub enum ShapeSource<BasicShape, ReferenceBox, Url> {
+    #[animation(error)]
     Url(Url),
-    Shape(BasicShape, Option<ReferenceBox>),
+    Shape(
+        BasicShape,
+        #[animation(constant)]
+        Option<ReferenceBox>,
+    ),
+    #[animation(error)]
     Box(ReferenceBox),
+    #[animation(error)]
     None,
 }
 
@@ -94,10 +101,13 @@ pub struct Ellipse<H, V, LengthOrPercentage> {
 /// https://drafts.csswg.org/css-shapes/#typedef-shape-radius
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, ComputeSquaredDistance, Copy, Debug, PartialEq, ToComputedValue, ToCss)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug, PartialEq)]
+#[derive(ToComputedValue, ToCss)]
 pub enum ShapeRadius<LengthOrPercentage> {
     Length(LengthOrPercentage),
+    #[animation(error)]
     ClosestSide,
+    #[animation(error)]
     FarthestSide,
 }
 
@@ -123,29 +133,6 @@ define_css_keyword_enum!(FillRule:
 );
 add_impls_for_keyword_enum!(FillRule);
 
-// FIXME(nox): This should be derivable, but we need to implement Animate
-// on the T types.
-impl<B, T, U> Animate for ShapeSource<B, T, U>
-where
-    B: Animate,
-    T: Clone + PartialEq,
-{
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (
-                &ShapeSource::Shape(ref this, ref this_box),
-                &ShapeSource::Shape(ref other, ref other_box),
-            ) if this_box == other_box => {
-                Ok(ShapeSource::Shape(
-                    this.animate(other, procedure)?,
-                    this_box.clone(),
-                ))
-            },
-            _ => Err(()),
-        }
-    }
-}
-
 // FIXME(nox): Implement ComputeSquaredDistance for T types and stop
 // using PartialEq here, this will let us derive this impl.
 impl<B, T, U> ComputeSquaredDistance for ShapeSource<B, T, U>
@@ -191,20 +178,6 @@ impl<L> ToCss for InsetRect<L>
     }
 }
 
-impl<L> Animate for ShapeRadius<L>
-where
-    L: Animate,
-{
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (&ShapeRadius::Length(ref this), &ShapeRadius::Length(ref other)) => {
-                Ok(ShapeRadius::Length(this.animate(other, procedure)?))
-            },
-            _ => Err(()),
-        }
-    }
-}
-
 impl<L> Default for ShapeRadius<L> {
     #[inline]
     fn default() -> Self { ShapeRadius::ClosestSide }
diff --git a/components/style/values/generics/effects.rs b/components/style/values/generics/effects.rs
index 3004c21f9b2..5d95ae19a8f 100644
--- a/components/style/values/generics/effects.rs
+++ b/components/style/values/generics/effects.rs
@@ -11,13 +11,15 @@ use values::specified::url::SpecifiedUrl;
 
 /// A generic value for a single `box-shadow`.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToAnimatedValue)]
+#[derive(Animate, Clone, Debug, HasViewportPercentage, PartialEq)]
+#[derive(ToAnimatedValue, ToAnimatedZero)]
 pub struct BoxShadow<Color, SizeLength, BlurShapeLength, ShapeLength> {
     /// The base shadow.
     pub base: SimpleShadow<Color, SizeLength, BlurShapeLength>,
     /// The spread radius.
     pub spread: ShapeLength,
     /// Whether this is an inset box shadow.
+    #[animation(constant)]
     pub inset: bool,
 }
 
diff --git a/components/style/values/generics/grid.rs b/components/style/values/generics/grid.rs
index 91b1e922d14..87f427f6cb3 100644
--- a/components/style/values/generics/grid.rs
+++ b/components/style/values/generics/grid.rs
@@ -10,7 +10,7 @@ use parser::{Parse, ParserContext};
 use std::{fmt, mem, usize};
 use style_traits::{ToCss, ParseError, StyleParseError};
 use values::{CSSFloat, CustomIdent, serialize_dimension};
-use values::computed::{ComputedValueAsSpecified, Context, ToComputedValue};
+use values::computed::ComputedValueAsSpecified;
 use values::specified::Integer;
 use values::specified::grid::parse_line_names;
 
@@ -137,13 +137,14 @@ define_css_keyword_enum!{ TrackKeyword:
     "max-content" => MaxContent,
     "min-content" => MinContent
 }
+impl ComputedValueAsSpecified for TrackKeyword {}
 
-#[derive(Clone, Debug, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 /// A track breadth for explicit grid track sizing. It's generic solely to
 /// avoid re-implementing it for the computed type.
 ///
 /// https://drafts.csswg.org/css-grid/#typedef-track-breadth
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, PartialEq, ToComputedValue)]
 pub enum TrackBreadth<L> {
     /// The generic type is almost always a non-negative `<length-percentage>`
     Breadth(L),
@@ -176,35 +177,12 @@ impl<L: ToCss> ToCss for TrackBreadth<L> {
     }
 }
 
-impl<L: ToComputedValue> ToComputedValue for TrackBreadth<L> {
-    type ComputedValue = TrackBreadth<L::ComputedValue>;
-
-    #[inline]
-    fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
-        match *self {
-            TrackBreadth::Breadth(ref lop) => TrackBreadth::Breadth(lop.to_computed_value(context)),
-            TrackBreadth::Flex(fr) => TrackBreadth::Flex(fr),
-            TrackBreadth::Keyword(k) => TrackBreadth::Keyword(k),
-        }
-    }
-
-    #[inline]
-    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
-        match *computed {
-            TrackBreadth::Breadth(ref lop) =>
-                TrackBreadth::Breadth(ToComputedValue::from_computed_value(lop)),
-            TrackBreadth::Flex(fr) => TrackBreadth::Flex(fr),
-            TrackBreadth::Keyword(k) => TrackBreadth::Keyword(k),
-        }
-    }
-}
-
 /// A `<track-size>` type for explicit grid track sizing. Like `<track-breadth>`, this is
 /// generic only to avoid code bloat. It only takes `<length-percentage>`
 ///
 /// https://drafts.csswg.org/css-grid/#typedef-track-size
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
 pub enum TrackSize<L> {
     /// A flexible `<track-breadth>`
     Breadth(TrackBreadth<L>),
@@ -286,39 +264,6 @@ impl<L: ToCss> ToCss for TrackSize<L> {
     }
 }
 
-impl<L: ToComputedValue> ToComputedValue for TrackSize<L> {
-    type ComputedValue = TrackSize<L::ComputedValue>;
-
-    #[inline]
-    fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
-        match *self {
-            TrackSize::Breadth(ref b) => match *b {
-                // <flex> outside `minmax()` expands to `mimmax(auto, <flex>)`
-                // https://drafts.csswg.org/css-grid/#valdef-grid-template-columns-flex
-                TrackBreadth::Flex(f) =>
-                    TrackSize::Minmax(TrackBreadth::Keyword(TrackKeyword::Auto), TrackBreadth::Flex(f)),
-                _ => TrackSize::Breadth(b.to_computed_value(context)),
-            },
-            TrackSize::Minmax(ref b_1, ref b_2) =>
-                TrackSize::Minmax(b_1.to_computed_value(context), b_2.to_computed_value(context)),
-            TrackSize::FitContent(ref lop) => TrackSize::FitContent(lop.to_computed_value(context)),
-        }
-    }
-
-    #[inline]
-    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
-        match *computed {
-            TrackSize::Breadth(ref b) =>
-                TrackSize::Breadth(ToComputedValue::from_computed_value(b)),
-            TrackSize::Minmax(ref b_1, ref b_2) =>
-                TrackSize::Minmax(ToComputedValue::from_computed_value(b_1),
-                                  ToComputedValue::from_computed_value(b_2)),
-            TrackSize::FitContent(ref lop) =>
-                TrackSize::FitContent(ToComputedValue::from_computed_value(lop)),
-        }
-    }
-}
-
 /// Helper function for serializing identifiers with a prefix and suffix, used
 /// for serializing <line-names> (in grid).
 pub fn concat_serialize_idents<W>(prefix: &str, suffix: &str,
@@ -382,8 +327,8 @@ no_viewport_percentage!(RepeatCount);
 ///
 /// It can also hold `repeat()` function parameters, which expands into the respective
 /// values in its computed form.
-#[derive(Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, PartialEq, ToComputedValue)]
 pub struct TrackRepeat<L> {
     /// The number of times for the value to be repeated (could also be `auto-fit` or `auto-fill`)
     pub count: RepeatCount,
@@ -392,6 +337,7 @@ pub struct TrackRepeat<L> {
     /// If there's no `<line-names>`, then it's represented by an empty vector.
     /// For N `<track-size>` values, there will be N+1 `<line-names>`, and so this vector's
     /// length is always one value more than that of the `<track-size>`.
+    #[compute(clone)]
     pub line_names: Box<[Box<[CustomIdent]>]>,
     /// `<track-size>` values.
     pub track_sizes: Vec<TrackSize<L>>,
@@ -479,31 +425,6 @@ impl<L: Clone> TrackRepeat<L> {
         }
     }
 }
-impl<L: ToComputedValue> ToComputedValue for TrackRepeat<L> {
-    type ComputedValue = TrackRepeat<L::ComputedValue>;
-
-    #[inline]
-    fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
-        TrackRepeat {
-            count: self.count,
-            track_sizes: self.track_sizes.iter()
-                                         .map(|val| val.to_computed_value(context))
-                                         .collect(),
-            line_names: self.line_names.clone(),
-        }
-    }
-
-    #[inline]
-    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
-        TrackRepeat {
-            count: computed.count,
-            track_sizes: computed.track_sizes.iter()
-                                             .map(ToComputedValue::from_computed_value)
-                                             .collect(),
-            line_names: computed.line_names.clone(),
-        }
-    }
-}
 
 /// The type of a `<track-list>` as determined during parsing.
 ///
@@ -533,8 +454,8 @@ impl ComputedValueAsSpecified for TrackListType {}
 /// A grid `<track-list>` type.
 ///
 /// https://drafts.csswg.org/css-grid/#typedef-track-list
-#[derive(Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, PartialEq, ToComputedValue)]
 pub struct TrackList<T> {
     /// The type of this `<track-list>` (auto, explicit or general).
     ///
@@ -548,6 +469,7 @@ pub struct TrackList<T> {
     /// If there's no `<line-names>`, then it's represented by an empty vector.
     /// For N values, there will be N+1 `<line-names>`, and so this vector's
     /// length is always one value more than that of the `<track-size>`.
+    #[compute(clone)]
     pub line_names: Box<[Box<[CustomIdent]>]>,
     /// `<auto-repeat>` value. There can only be one `<auto-repeat>` in a TrackList.
     pub auto_repeat: Option<TrackRepeat<T>>,
@@ -698,8 +620,8 @@ no_viewport_percentage!(LineNameList);
 /// Variants for `<grid-template-rows> | <grid-template-columns>`
 /// Subgrid deferred to Level 2 spec due to lack of implementation.
 /// But it's implemented in gecko, so we have to as well.
-#[derive(Clone, Debug, PartialEq, ToCss)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, PartialEq, ToComputedValue, ToCss)]
 pub enum GridTemplateComponent<L> {
     /// `none` value.
     None,
diff --git a/components/style/values/generics/image.rs b/components/style/values/generics/image.rs
index 867364e3df3..3e977759d70 100644
--- a/components/style/values/generics/image.rs
+++ b/components/style/values/generics/image.rs
@@ -35,16 +35,18 @@ pub enum Image<Gradient, MozImageRect, ImageUrl> {
 
 /// A CSS gradient.
 /// https://drafts.csswg.org/css-images/#gradients
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
 pub struct Gradient<LineDirection, Length, LengthOrPercentage, Position, Color, Angle> {
     /// Gradients can be linear or radial.
     pub kind: GradientKind<LineDirection, Length, LengthOrPercentage, Position, Angle>,
     /// The color stops and interpolation hints.
     pub items: Vec<GradientItem<Color, LengthOrPercentage>>,
     /// True if this is a repeating gradient.
+    #[compute(clone)]
     pub repeating: bool,
     /// Compatibility mode.
+    #[compute(clone)]
     pub compat_mode: CompatMode,
 }
 
@@ -61,8 +63,8 @@ pub enum CompatMode {
 }
 
 /// A gradient kind.
-#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
 pub enum GradientKind<LineDirection, Length, LengthOrPercentage, Position, Angle> {
     /// A linear gradient.
     Linear(LineDirection),
diff --git a/components/style/values/generics/svg.rs b/components/style/values/generics/svg.rs
index bc3cb5fda18..87b0a74bc23 100644
--- a/components/style/values/generics/svg.rs
+++ b/components/style/values/generics/svg.rs
@@ -16,7 +16,8 @@ use values::distance::{ComputeSquaredDistance, SquaredDistance};
 ///
 /// https://www.w3.org/TR/SVG2/painting.html#SpecifyingPaint
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToAnimatedValue, ToComputedValue, ToCss)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Debug, PartialEq)]
+#[derive(ToAnimatedValue, ToComputedValue, ToCss)]
 pub struct SVGPaint<ColorType, UrlPaintServer> {
     /// The paint source
     pub kind: SVGPaintKind<ColorType, UrlPaintServer>,
@@ -30,13 +31,15 @@ pub struct SVGPaint<ColorType, UrlPaintServer> {
 /// to have a fallback, Gecko lets the context
 /// properties have a fallback as well.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToAnimatedValue, ToComputedValue, ToCss)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Debug, PartialEq)]
+#[derive(ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
 pub enum SVGPaintKind<ColorType, UrlPaintServer> {
     /// `none`
     None,
     /// `<color>`
     Color(ColorType),
     /// `url(...)`
+    #[animation(error)]
     PaintServer(UrlPaintServer),
     /// `context-fill`
     ContextFill,
diff --git a/components/style/values/specified/grid.rs b/components/style/values/specified/grid.rs
index 92a442a1cf0..72ce404d25c 100644
--- a/components/style/values/specified/grid.rs
+++ b/components/style/values/specified/grid.rs
@@ -11,7 +11,6 @@ use std::ascii::AsciiExt;
 use std::mem;
 use style_traits::{HasViewportPercentage, ParseError, StyleParseError};
 use values::{CSSFloat, CustomIdent};
-use values::computed::{self, Context, ToComputedValue};
 use values::generics::grid::{GridTemplateComponent, RepeatCount, TrackBreadth, TrackKeyword, TrackRepeat};
 use values::generics::grid::{LineNameList, TrackSize, TrackList, TrackListType};
 use values::specified::LengthOrPercentage;
@@ -286,42 +285,6 @@ impl HasViewportPercentage for TrackList<LengthOrPercentage> {
     }
 }
 
-
-impl ToComputedValue for TrackList<LengthOrPercentage> {
-    type ComputedValue = TrackList<computed::LengthOrPercentage>;
-
-    #[inline]
-    fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
-        let mut values = Vec::with_capacity(self.values.len() + 1);
-        for value in self.values.iter().map(|val| val.to_computed_value(context)) {
-            values.push(value);
-        }
-
-        TrackList {
-            list_type: self.list_type.to_computed_value(context),
-            values: values,
-            line_names: self.line_names.clone(),
-            auto_repeat: self.auto_repeat.clone().map(|repeat| repeat.to_computed_value(context)),
-        }
-    }
-
-    #[inline]
-    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
-        let mut values = Vec::with_capacity(computed.values.len() + 1);
-        for value in computed.values.iter().map(ToComputedValue::from_computed_value) {
-            values.push(value);
-        }
-
-        TrackList {
-            list_type: computed.list_type,
-            values: values,
-            line_names: computed.line_names.clone(),
-            auto_repeat: computed.auto_repeat.clone().map(|ref repeat| TrackRepeat::from_computed_value(repeat)),
-        }
-    }
-}
-
-
 impl Parse for GridTemplateComponent<LengthOrPercentage> {
     // FIXME: Derive Parse (probably with None_)
     fn parse<'i, 't>(context: &ParserContext, input: &mut Parser<'i, 't>) -> Result<Self, ParseError<'i>> {
@@ -354,27 +317,3 @@ impl HasViewportPercentage for GridTemplateComponent<LengthOrPercentage> {
         }
     }
 }
-
-impl ToComputedValue for GridTemplateComponent<LengthOrPercentage> {
-    type ComputedValue = GridTemplateComponent<computed::LengthOrPercentage>;
-
-    #[inline]
-    fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
-        match *self {
-            GridTemplateComponent::None => GridTemplateComponent::None,
-            GridTemplateComponent::TrackList(ref l) => GridTemplateComponent::TrackList(l.to_computed_value(context)),
-            GridTemplateComponent::Subgrid(ref n) => GridTemplateComponent::Subgrid(n.to_computed_value(context)),
-        }
-    }
-
-    #[inline]
-    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
-        match *computed {
-            GridTemplateComponent::None => GridTemplateComponent::None,
-            GridTemplateComponent::TrackList(ref l) =>
-                GridTemplateComponent::TrackList(ToComputedValue::from_computed_value(l)),
-            GridTemplateComponent::Subgrid(ref n) =>
-                GridTemplateComponent::Subgrid(ToComputedValue::from_computed_value(n)),
-        }
-    }
-}
diff --git a/components/style_derive/animate.rs b/components/style_derive/animate.rs
index 94d14336087..d4fe5c94a53 100644
--- a/components/style_derive/animate.rs
+++ b/components/style_derive/animate.rs
@@ -3,21 +3,22 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use cg;
-use quote;
-use syn;
+use quote::Tokens;
+use syn::{DeriveInput, Path};
 
-pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
+pub fn derive(input: DeriveInput) -> Tokens {
     let name = &input.ident;
     let trait_path = &["values", "animated", "Animate"];
     let (impl_generics, ty_generics, mut where_clause) =
         cg::trait_parts(&input, trait_path);
 
+    let input_attrs = cg::parse_input_attrs::<AnimateInputAttrs>(&input);
     let variants = cg::variants(&input);
     let mut match_body = quote!();
     let mut append_error_clause = variants.len() > 1;
     match_body.append_all(variants.iter().flat_map(|variant| {
-        let attrs = cg::parse_variant_attrs::<AnimateAttrs>(variant);
-        if attrs.error {
+        let variant_attrs = cg::parse_variant_attrs::<AnimationVariantAttrs>(variant);
+        if variant_attrs.error {
             append_error_clause = true;
             return None;
         }
@@ -28,11 +29,32 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
         let mut computations = quote!();
         let iter = result_info.iter().zip(this_info.iter().zip(&other_info));
         computations.append_all(iter.map(|(result, (this, other))| {
-            where_clause.predicates.push(
-                cg::where_predicate(this.field.ty.clone(), trait_path),
-            );
-            quote! {
-                let #result = ::values::animated::Animate::animate(#this, #other, procedure)?;
+            let field_attrs = cg::parse_field_attrs::<AnimationFieldAttrs>(&result.field);
+            if field_attrs.constant {
+                if cg::is_parameterized(&result.field.ty, where_clause.params, None) {
+                    where_clause.inner.predicates.push(cg::where_predicate(
+                        result.field.ty.clone(),
+                        &["std", "cmp", "PartialEq"],
+                        None,
+                    ));
+                    where_clause.inner.predicates.push(cg::where_predicate(
+                        result.field.ty.clone(),
+                        &["std", "clone", "Clone"],
+                        None,
+                    ));
+                }
+                quote! {
+                    if #this != #other {
+                        return Err(());
+                    }
+                    let #result = ::std::clone::Clone::clone(#this);
+                }
+            } else {
+                where_clause.add_trait_bound(&result.field.ty);
+                quote! {
+                    let #result =
+                        ::values::animated::Animate::animate(#this, #other, procedure)?;
+                }
             }
         }));
         Some(quote! {
@@ -44,7 +66,13 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
     }));
 
     if append_error_clause {
-        match_body = quote! { #match_body, _ => Err(()), };
+        if let Some(fallback) = input_attrs.fallback {
+            match_body.append(quote! {
+                (this, other) => #fallback(this, other, procedure)
+            });
+        } else {
+            match_body.append(quote! { _ => Err(()) });
+        }
     }
 
     quote! {
@@ -64,8 +92,20 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
     }
 }
 
-#[derive(Default, FromVariant)]
 #[darling(attributes(animate), default)]
-pub struct AnimateAttrs {
+#[derive(Default, FromDeriveInput)]
+struct AnimateInputAttrs {
+    fallback: Option<Path>,
+}
+
+#[darling(attributes(animation), default)]
+#[derive(Default, FromVariant)]
+pub struct AnimationVariantAttrs {
     pub error: bool,
 }
+
+#[darling(attributes(animation), default)]
+#[derive(Default, FromField)]
+pub struct AnimationFieldAttrs {
+    pub constant: bool,
+}
diff --git a/components/style_derive/cg.rs b/components/style_derive/cg.rs
index b670da16737..6937d5836d2 100644
--- a/components/style_derive/cg.rs
+++ b/components/style_derive/cg.rs
@@ -2,17 +2,86 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use darling::FromVariant;
-use quote::Tokens;
+use darling::{FromDeriveInput, FromField, FromVariant};
+use quote::{ToTokens, Tokens};
 use std::borrow::Cow;
+use std::collections::HashSet;
 use std::iter;
-use syn::{AngleBracketedParameterData, Body, DeriveInput, Ident, ImplGenerics};
-use syn::{Path, PathParameters, PathSegment, PolyTraitRef, QSelf};
-use syn::{TraitBoundModifier, Ty, TyGenerics, TyParam, TyParamBound, TypeBinding};
-use syn::{Variant, WhereBoundPredicate, WhereClause, WherePredicate};
+use syn::{self, AngleBracketedParameterData, Body, DeriveInput, Field, Ident};
+use syn::{ImplGenerics, Path, PathParameters, PathSegment, PolyTraitRef};
+use syn::{QSelf, TraitBoundModifier, Ty, TyGenerics, TyParam, TyParamBound};
+use syn::{TypeBinding, Variant, WhereBoundPredicate, WherePredicate};
 use syn::visit::{self, Visitor};
 use synstructure::{self, BindOpts, BindStyle, BindingInfo};
 
+pub struct WhereClause<'input, 'path> {
+    pub inner: syn::WhereClause,
+    pub params: &'input [TyParam],
+    trait_path: &'path [&'path str],
+    trait_output: Option<&'path str>,
+    bounded_types: HashSet<Ty>,
+}
+
+impl<'input, 'path> ToTokens for WhereClause<'input, 'path> {
+    fn to_tokens(&self, tokens: &mut Tokens) {
+        self.inner.to_tokens(tokens);
+    }
+}
+
+impl<'input, 'path> WhereClause<'input, 'path> {
+    pub fn add_trait_bound(&mut self, ty: &Ty) {
+        let trait_path = self.trait_path;
+        let params = self.params;
+        let mut found = self.trait_output.map(|_| HashSet::new());
+        if self.bounded_types.contains(&ty) {
+            return;
+        }
+        if !is_parameterized(&ty, params, found.as_mut()) {
+            return;
+        }
+        self.bounded_types.insert(ty.clone());
+
+        let output = if let Some(output) = self.trait_output {
+            output
+        } else {
+            self.inner.predicates.push(where_predicate(ty.clone(), trait_path, None));
+            return;
+        };
+
+        if let Ty::Path(None, ref path) = *ty {
+            if path_to_ident(path).is_some() {
+                self.inner.predicates.push(where_predicate(ty.clone(), trait_path, None));
+                return;
+            }
+        }
+
+        let output_type = map_type_params(ty, params, &mut |ident| {
+            let ty = Ty::Path(None, ident.clone().into());
+            fmap_output_type(ty, trait_path, output)
+        });
+
+        let pred = where_predicate(
+            ty.clone(),
+            trait_path,
+            Some((output, output_type)),
+        );
+
+        self.inner.predicates.push(pred);
+
+        if let Some(found) = found {
+            for ident in found {
+                let ty = Ty::Path(None, ident.into());
+                if !self.bounded_types.contains(&ty) {
+                    self.bounded_types.insert(ty.clone());
+                    self.inner.predicates.push(
+                        where_predicate(ty, trait_path, None),
+                    );
+                };
+            }
+        }
+    }
+}
+
 pub fn fmap_match<F>(
     input: &DeriveInput,
     bind_style: BindStyle,
@@ -35,23 +104,36 @@ where
     })
 }
 
-pub fn fmap_trait_parts<'a>(
-    input: &'a DeriveInput,
+fn fmap_output_type(
+    ty: Ty,
     trait_path: &[&str],
     trait_output: &str,
-) -> (ImplGenerics<'a>, TyGenerics<'a>, WhereClause, Path) {
-    let (impl_generics, ty_generics, where_clause) = trait_parts(input, trait_path);
+) -> Ty {
+    Ty::Path(
+        Some(QSelf {
+            ty: Box::new(ty),
+            position: trait_path.len(),
+        }),
+        path(trait_path.iter().chain(iter::once(&trait_output))),
+    )
+}
+
+pub fn fmap_trait_parts<'input, 'path>(
+    input: &'input DeriveInput,
+    trait_path: &'path [&'path str],
+    trait_output: &'path str,
+) -> (ImplGenerics<'input>, TyGenerics<'input>, WhereClause<'input, 'path>, Path) {
+    let (impl_generics, ty_generics, mut where_clause) = trait_parts(input, trait_path);
+    where_clause.trait_output = Some(trait_output);
     let output_ty = PathSegment {
         ident: input.ident.clone(),
         parameters: PathParameters::AngleBracketed(AngleBracketedParameterData {
             lifetimes: input.generics.lifetimes.iter().map(|l| l.lifetime.clone()).collect(),
             types: input.generics.ty_params.iter().map(|ty| {
-                Ty::Path(
-                    Some(QSelf {
-                        ty: Box::new(Ty::Path(None, ty.ident.clone().into())),
-                        position: trait_path.len(),
-                    }),
-                    path(trait_path.iter().chain(iter::once(&trait_output))),
+                fmap_output_type(
+                    Ty::Path(None, ty.ident.clone().into()),
+                    trait_path,
+                    trait_output,
                 )
             }).collect(),
             .. Default::default()
@@ -60,72 +142,106 @@ pub fn fmap_trait_parts<'a>(
     (impl_generics, ty_generics, where_clause, output_ty)
 }
 
-fn fmap_trait_where_predicate(
-    bounded_ty: Ty,
-    trait_path: &[&str],
-    trait_output: Option<(&str, Ty)>,
-) -> WherePredicate {
-    WherePredicate::BoundPredicate(WhereBoundPredicate {
-        bound_lifetimes: vec![],
-        bounded_ty,
-        bounds: vec![TyParamBound::Trait(
-            PolyTraitRef {
-                bound_lifetimes: vec![],
-                trait_ref: fmap_trait_ref(trait_path, trait_output),
-            },
-            TraitBoundModifier::None
-        )],
-    })
-}
-
-fn fmap_trait_ref(path: &[&str], output: Option<(&str, Ty)>) -> Path {
-    let (name, parent) = path.split_last().unwrap();
-    let last_segment = PathSegment {
-        ident: (*name).into(),
-        parameters: PathParameters::AngleBracketed(
-            AngleBracketedParameterData {
-                bindings: output.into_iter().map(|(param, ty)| {
-                    TypeBinding { ident: param.into(), ty }
-                }).collect(),
-                .. Default::default()
-            }
-        )
-    };
-    Path {
-        global: true,
-        segments: {
-            parent
-                .iter()
-                .cloned()
-                .map(Into::into)
-                .chain(iter::once(last_segment))
-                .collect()
-        },
-    }
-}
-
-pub fn is_parameterized(ty: &Ty, params: &[TyParam]) -> bool {
-    struct IsParameterized<'a> {
+pub fn is_parameterized(
+    ty: &Ty,
+    params: &[TyParam],
+    found: Option<&mut HashSet<Ident>>,
+) -> bool {
+    struct IsParameterized<'a, 'b> {
         params: &'a [TyParam],
         has_free: bool,
+        found: Option<&'b mut HashSet<Ident>>,
     }
 
-    impl<'a> Visitor for IsParameterized<'a> {
+    impl<'a, 'b> Visitor for IsParameterized<'a, 'b> {
         fn visit_path(&mut self, path: &Path) {
-            if !path.global && path.segments.len() == 1 {
-                if self.params.iter().any(|param| param.ident == path.segments[0].ident) {
+            if let Some(ident) = path_to_ident(path) {
+                if self.params.iter().any(|param| param.ident == ident) {
                     self.has_free = true;
+                    if let Some(ref mut found) = self.found {
+                        found.insert(ident.clone());
+                    }
                 }
             }
             visit::walk_path(self, path);
         }
     }
 
-    let mut visitor = IsParameterized { params: params, has_free: false };
+    let mut visitor = IsParameterized { params, has_free: false, found };
     visitor.visit_ty(ty);
     visitor.has_free
 }
 
+pub fn map_type_params<F>(ty: &Ty, params: &[TyParam], f: &mut F) -> Ty
+where
+    F: FnMut(&Ident) -> Ty,
+{
+    match *ty {
+        Ty::Slice(ref ty) => Ty::Slice(Box::new(map_type_params(ty, params, f))),
+        Ty::Array(ref ty, ref expr) => {
+            Ty::Array(Box::new(map_type_params(ty, params, f)), expr.clone())
+        },
+        Ty::Never => Ty::Never,
+        Ty::Tup(ref items) => {
+            Ty::Tup(items.iter().map(|ty| map_type_params(ty, params, f)).collect())
+        },
+        Ty::Path(None, ref path) => {
+            if let Some(ident) = path_to_ident(path) {
+                if params.iter().any(|param| param.ident == ident) {
+                    return f(ident);
+                }
+            }
+            Ty::Path(None, map_type_params_in_path(path, params, f))
+        }
+        Ty::Path(ref qself, ref path) => {
+            Ty::Path(
+                qself.as_ref().map(|qself| {
+                    QSelf {
+                        ty: Box::new(map_type_params(&qself.ty, params, f)),
+                        position: qself.position,
+                    }
+                }),
+                map_type_params_in_path(path, params, f),
+            )
+        },
+        Ty::Paren(ref ty) => Ty::Paren(Box::new(map_type_params(ty, params, f))),
+        ref ty => panic!("type {:?} cannot be mapped yet", ty),
+    }
+}
+
+fn map_type_params_in_path<F>(path: &Path, params: &[TyParam], f: &mut F) -> Path
+where
+    F: FnMut(&Ident) -> Ty,
+{
+    Path {
+        global: path.global,
+        segments: path.segments.iter().map(|segment| {
+            PathSegment {
+                ident: segment.ident.clone(),
+                parameters: match segment.parameters {
+                    PathParameters::AngleBracketed(ref data) => {
+                        PathParameters::AngleBracketed(AngleBracketedParameterData {
+                            lifetimes: data.lifetimes.clone(),
+                            types: data.types.iter().map(|ty| {
+                                map_type_params(ty, params, f)
+                            }).collect(),
+                            bindings: data.bindings.iter().map(|binding| {
+                                TypeBinding {
+                                    ident: binding.ident.clone(),
+                                    ty: map_type_params(&binding.ty, params, f),
+                                }
+                            }).collect(),
+                        })
+                    },
+                    ref parameters => {
+                        panic!("parameters {:?} cannot be mapped yet", parameters)
+                    },
+                },
+            }
+        }).collect(),
+    }
+}
+
 pub fn path<S>(segments: S) -> Path
 where
     S: IntoIterator,
@@ -137,13 +253,46 @@ where
     }
 }
 
+fn path_to_ident(path: &Path) -> Option<&Ident> {
+    match *path {
+        Path { global: false, ref segments } if segments.len() == 1 => {
+            if segments[0].parameters.is_empty() {
+                Some(&segments[0].ident)
+            } else {
+                None
+            }
+        },
+        _ => None,
+    }
+}
+
+pub fn parse_field_attrs<A>(field: &Field) -> A
+where
+    A: FromField,
+{
+    match A::from_field(field) {
+        Ok(attrs) => attrs,
+        Err(e) => panic!("failed to parse field attributes: {}", e),
+    }
+}
+
+pub fn parse_input_attrs<A>(input: &DeriveInput) -> A
+where
+    A: FromDeriveInput,
+{
+    match A::from_derive_input(input) {
+        Ok(attrs) => attrs,
+        Err(e) => panic!("failed to parse input attributes: {}", e),
+    }
+}
+
 pub fn parse_variant_attrs<A>(variant: &Variant) -> A
 where
     A: FromVariant,
 {
     match A::from_variant(variant) {
         Ok(attrs) => attrs,
-        Err(e) => panic!("failed to parse attributes: {}", e),
+        Err(e) => panic!("failed to parse variant attributes: {}", e),
     }
 }
 
@@ -159,22 +308,47 @@ pub fn ref_pattern<'a>(
     )
 }
 
-pub fn trait_parts<'a>(
-    input: &'a DeriveInput,
-    trait_path: &[&str],
-) -> (ImplGenerics<'a>, TyGenerics<'a>, WhereClause) {
+pub fn trait_parts<'input, 'path>(
+    input: &'input DeriveInput,
+    trait_path: &'path [&'path str],
+) -> (ImplGenerics<'input>, TyGenerics<'input>, WhereClause<'input, 'path>) {
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
-    let mut where_clause = where_clause.clone();
-    for param in &input.generics.ty_params {
-        where_clause.predicates.push(fmap_trait_where_predicate(
-            Ty::Path(None, param.ident.clone().into()),
-            trait_path,
-            None,
-        ));
-    }
+    let where_clause = WhereClause {
+        inner: where_clause.clone(),
+        params: &input.generics.ty_params,
+        trait_path,
+        trait_output: None,
+        bounded_types: HashSet::new()
+    };
     (impl_generics, ty_generics, where_clause)
 }
 
+fn trait_ref(path: &[&str], output: Option<(&str, Ty)>) -> Path {
+    let (name, parent) = path.split_last().unwrap();
+    let last_segment = PathSegment {
+        ident: (*name).into(),
+        parameters: PathParameters::AngleBracketed(
+            AngleBracketedParameterData {
+                bindings: output.into_iter().map(|(param, ty)| {
+                    TypeBinding { ident: param.into(), ty }
+                }).collect(),
+                .. Default::default()
+            }
+        )
+    };
+    Path {
+        global: true,
+        segments: {
+            parent
+                .iter()
+                .cloned()
+                .map(Into::into)
+                .chain(iter::once(last_segment))
+                .collect()
+        },
+    }
+}
+
 pub fn value<'a>(
     name: &Ident,
     variant: &'a Variant,
@@ -215,16 +389,20 @@ pub fn variants(input: &DeriveInput) -> Cow<[Variant]> {
     }
 }
 
-pub fn where_predicate(ty: Ty, segments: &[&str]) -> WherePredicate {
+pub fn where_predicate(
+    bounded_ty: Ty,
+    trait_path: &[&str],
+    trait_output: Option<(&str, Ty)>,
+) -> WherePredicate {
     WherePredicate::BoundPredicate(WhereBoundPredicate {
         bound_lifetimes: vec![],
-        bounded_ty: ty,
+        bounded_ty,
         bounds: vec![TyParamBound::Trait(
             PolyTraitRef {
                 bound_lifetimes: vec![],
-                trait_ref: path(segments),
+                trait_ref: trait_ref(trait_path, trait_output),
             },
-            TraitBoundModifier::None,
+            TraitBoundModifier::None
         )],
     })
 }
diff --git a/components/style_derive/compute_squared_distance.rs b/components/style_derive/compute_squared_distance.rs
index 860e568ea88..cbdd9c62acc 100644
--- a/components/style_derive/compute_squared_distance.rs
+++ b/components/style_derive/compute_squared_distance.rs
@@ -2,22 +2,23 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use animate::AnimateAttrs;
+use animate::AnimationVariantAttrs;
 use cg;
-use quote;
-use syn;
+use quote::Tokens;
+use syn::{DeriveInput, Path};
 
-pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
+pub fn derive(input: DeriveInput) -> Tokens {
     let name = &input.ident;
     let trait_path = &["values", "distance", "ComputeSquaredDistance"];
     let (impl_generics, ty_generics, mut where_clause) =
         cg::trait_parts(&input, trait_path);
 
+    let input_attrs = cg::parse_input_attrs::<DistanceInputAttrs>(&input);
     let variants = cg::variants(&input);
     let mut match_body = quote!();
     let mut append_error_clause = variants.len() > 1;
     match_body.append_all(variants.iter().map(|variant| {
-        let attrs = cg::parse_variant_attrs::<AnimateAttrs>(variant);
+        let attrs = cg::parse_variant_attrs::<AnimationVariantAttrs>(variant);
         if attrs.error {
             append_error_clause = true;
             return None;
@@ -30,9 +31,7 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
         } else {
             let mut sum = quote!();
             sum.append_separated(this_info.iter().zip(&other_info).map(|(this, other)| {
-                where_clause.predicates.push(
-                    cg::where_predicate(this.field.ty.clone(), trait_path),
-                );
+                where_clause.add_trait_bound(&this.field.ty);
                 quote! {
                     ::values::distance::ComputeSquaredDistance::compute_squared_distance(#this, #other)?
                 }
@@ -47,7 +46,13 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
     }));
 
     if append_error_clause {
-        match_body = quote! { #match_body, _ => Err(()), };
+        if let Some(fallback) = input_attrs.fallback {
+            match_body.append(quote! {
+                (this, other) => #fallback(this, other)
+            });
+        } else {
+            match_body.append(quote! { _ => Err(()) });
+        }
     }
 
     quote! {
@@ -65,3 +70,9 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
         }
     }
 }
+
+#[darling(attributes(distance), default)]
+#[derive(Default, FromDeriveInput)]
+struct DistanceInputAttrs {
+    fallback: Option<Path>,
+}
diff --git a/components/style_derive/has_viewport_percentage.rs b/components/style_derive/has_viewport_percentage.rs
index 5f26d988b8a..18856576fb8 100644
--- a/components/style_derive/has_viewport_percentage.rs
+++ b/components/style_derive/has_viewport_percentage.rs
@@ -19,11 +19,10 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
             None => return Some(quote!(false)),
             Some(pair) => pair,
         };
+        where_clause.add_trait_bound(&first.field.ty);
         let mut expr = quote!(::style_traits::HasViewportPercentage::has_viewport_percentage(#first));
         for binding in rest {
-            where_clause.predicates.push(
-                cg::where_predicate(binding.field.ty.clone(), trait_path),
-            );
+            where_clause.add_trait_bound(&binding.field.ty);
             expr = quote!(#expr || ::style_traits::HasViewportPercentage::has_viewport_percentage(#binding));
         }
         Some(expr)
diff --git a/components/style_derive/lib.rs b/components/style_derive/lib.rs
index e589759f813..f98829e60bf 100644
--- a/components/style_derive/lib.rs
+++ b/components/style_derive/lib.rs
@@ -19,13 +19,13 @@ mod to_animated_zero;
 mod to_computed_value;
 mod to_css;
 
-#[proc_macro_derive(Animate, attributes(animation))]
+#[proc_macro_derive(Animate, attributes(animate, animation))]
 pub fn derive_animate(stream: TokenStream) -> TokenStream {
     let input = syn::parse_derive_input(&stream.to_string()).unwrap();
     animate::derive(input).to_string().parse().unwrap()
 }
 
-#[proc_macro_derive(ComputeSquaredDistance, attributes(animation))]
+#[proc_macro_derive(ComputeSquaredDistance, attributes(animation, distance))]
 pub fn derive_compute_squared_distance(stream: TokenStream) -> TokenStream {
     let input = syn::parse_derive_input(&stream.to_string()).unwrap();
     compute_squared_distance::derive(input).to_string().parse().unwrap()
@@ -43,13 +43,13 @@ pub fn derive_to_animated_value(stream: TokenStream) -> TokenStream {
     to_animated_value::derive(input).to_string().parse().unwrap()
 }
 
-#[proc_macro_derive(ToAnimatedZero)]
+#[proc_macro_derive(ToAnimatedZero, attributes(animation))]
 pub fn derive_to_animated_zero(stream: TokenStream) -> TokenStream {
     let input = syn::parse_derive_input(&stream.to_string()).unwrap();
     to_animated_zero::derive(input).to_string().parse().unwrap()
 }
 
-#[proc_macro_derive(ToComputedValue)]
+#[proc_macro_derive(ToComputedValue, attributes(compute))]
 pub fn derive_to_computed_value(stream: TokenStream) -> TokenStream {
     let input = syn::parse_derive_input(&stream.to_string()).unwrap();
     to_computed_value::derive(input).to_string().parse().unwrap()
diff --git a/components/style_derive/to_animated_value.rs b/components/style_derive/to_animated_value.rs
index 5a66ddf65a4..7865ba597e9 100644
--- a/components/style_derive/to_animated_value.rs
+++ b/components/style_derive/to_animated_value.rs
@@ -9,18 +9,16 @@ use synstructure::BindStyle;
 
 pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
     let name = &input.ident;
-    let (impl_generics, ty_generics, where_clause, animated_value_type) =
-        cg::fmap_trait_parts(
-            &input,
-            &["values", "animated", "ToAnimatedValue"],
-            "AnimatedValue",
-        );
+    let trait_path = &["values", "animated", "ToAnimatedValue"];
+    let (impl_generics, ty_generics, mut where_clause, animated_value_type) =
+        cg::fmap_trait_parts(&input, trait_path, "AnimatedValue");
 
-    let to_body = cg::fmap_match(&input, BindStyle::Move, |field| {
-        quote!(::values::animated::ToAnimatedValue::to_animated_value(#field))
+    let to_body = cg::fmap_match(&input, BindStyle::Move, |binding| {
+        where_clause.add_trait_bound(&binding.field.ty);
+        quote!(::values::animated::ToAnimatedValue::to_animated_value(#binding))
     });
-    let from_body = cg::fmap_match(&input, BindStyle::Move, |field| {
-        quote!(::values::animated::ToAnimatedValue::from_animated_value(#field))
+    let from_body = cg::fmap_match(&input, BindStyle::Move, |binding| {
+        quote!(::values::animated::ToAnimatedValue::from_animated_value(#binding))
     });
 
     quote! {
diff --git a/components/style_derive/to_animated_zero.rs b/components/style_derive/to_animated_zero.rs
index d9e67a8a9fe..be1210faa7a 100644
--- a/components/style_derive/to_animated_zero.rs
+++ b/components/style_derive/to_animated_zero.rs
@@ -2,20 +2,51 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use animate::{AnimationVariantAttrs, AnimationFieldAttrs};
 use cg;
 use quote;
 use syn;
-use synstructure::BindStyle;
+use synstructure::{self, BindStyle};
 
 pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
     let name = &input.ident;
-    let (impl_generics, ty_generics, where_clause) = cg::trait_parts(
-        &input,
-        &["values", "animated", "ToAnimatedZero"],
-    );
+    let trait_path = &["values", "animated", "ToAnimatedZero"];
+    let (impl_generics, ty_generics, mut where_clause) =
+        cg::trait_parts(&input, trait_path);
 
-    let to_body = cg::fmap_match(&input, BindStyle::Ref, |field| {
-        quote! { ::values::animated::ToAnimatedZero::to_animated_zero(#field)? }
+    let bind_opts = BindStyle::Ref.into();
+    let to_body = synstructure::each_variant(&input, &bind_opts, |bindings, variant| {
+        let attrs = cg::parse_variant_attrs::<AnimationVariantAttrs>(variant);
+        if attrs.error {
+            return Some(quote! { Err(()) });
+        }
+        let name = cg::variant_ctor(&input, variant);
+        let (mapped, mapped_bindings) = cg::value(&name, variant, "mapped");
+        let bindings_pairs = bindings.into_iter().zip(mapped_bindings);
+        let mut computations = quote!();
+        computations.append_all(bindings_pairs.map(|(binding, mapped_binding)| {
+            let field_attrs = cg::parse_field_attrs::<AnimationFieldAttrs>(&binding.field);
+            if field_attrs.constant {
+                if cg::is_parameterized(&binding.field.ty, where_clause.params, None) {
+                    where_clause.inner.predicates.push(cg::where_predicate(
+                        binding.field.ty.clone(),
+                        &["std", "clone", "Clone"],
+                        None,
+                    ));
+                }
+                quote! {
+                    let #mapped_binding = ::std::clone::Clone::clone(#binding);
+                }
+            } else {
+                where_clause.add_trait_bound(&binding.field.ty);
+                quote! {
+                    let #mapped_binding =
+                        ::values::animated::ToAnimatedZero::to_animated_zero(#binding)?;
+                }
+            }
+        }));
+        computations.append(quote! { Ok(#mapped) });
+        Some(computations)
     });
 
     quote! {
@@ -23,9 +54,9 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
             #[allow(unused_variables)]
             #[inline]
             fn to_animated_zero(&self) -> Result<Self, ()> {
-                Ok(match *self {
+                match *self {
                     #to_body
-                })
+                }
             }
         }
     }
diff --git a/components/style_derive/to_computed_value.rs b/components/style_derive/to_computed_value.rs
index 967f3320a37..7e4db276edd 100644
--- a/components/style_derive/to_computed_value.rs
+++ b/components/style_derive/to_computed_value.rs
@@ -3,24 +3,43 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use cg;
-use quote;
-use syn;
+use quote::Tokens;
+use syn::DeriveInput;
 use synstructure::BindStyle;
 
-pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
+pub fn derive(input: DeriveInput) -> Tokens {
     let name = &input.ident;
-    let (impl_generics, ty_generics, where_clause, computed_value_type) =
-        cg::fmap_trait_parts(
-            &input,
-            &["values", "computed", "ToComputedValue"],
-            "ComputedValue",
-        );
+    let trait_path = &["values", "computed", "ToComputedValue"];
+    let (impl_generics, ty_generics, mut where_clause, computed_value_type) =
+        cg::fmap_trait_parts(&input, trait_path, "ComputedValue");
 
-    let to_body = cg::fmap_match(&input, BindStyle::Ref, |field| {
-        quote!(::values::computed::ToComputedValue::to_computed_value(#field, context))
+    let to_body = cg::fmap_match(&input, BindStyle::Ref, |binding| {
+        let attrs = cg::parse_field_attrs::<ComputedValueAttrs>(&binding.field);
+        if attrs.clone {
+            if cg::is_parameterized(&binding.field.ty, where_clause.params, None) {
+                where_clause.inner.predicates.push(cg::where_predicate(
+                    binding.field.ty.clone(),
+                    &["std", "clone", "Clone"],
+                    None,
+                ));
+            }
+            quote! { ::std::clone::Clone::clone(#binding) }
+        } else {
+            where_clause.add_trait_bound(&binding.field.ty);
+            quote! {
+                ::values::computed::ToComputedValue::to_computed_value(#binding, context)
+            }
+        }
     });
-    let from_body = cg::fmap_match(&input, BindStyle::Ref, |field| {
-        quote!(::values::computed::ToComputedValue::from_computed_value(#field))
+    let from_body = cg::fmap_match(&input, BindStyle::Ref, |binding| {
+        let attrs = cg::parse_field_attrs::<ComputedValueAttrs>(&binding.field);
+        if attrs.clone {
+            quote! { ::std::clone::Clone::clone(#binding) }
+        } else {
+            quote! {
+                ::values::computed::ToComputedValue::from_computed_value(#binding)
+            }
+        }
     });
 
     quote! {
@@ -44,3 +63,9 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
         }
     }
 }
+
+#[darling(attributes(compute), default)]
+#[derive(Default, FromField)]
+struct ComputedValueAttrs {
+    clone: bool,
+}
diff --git a/components/style_derive/to_css.rs b/components/style_derive/to_css.rs
index 444c42e37da..6ae438ccbd2 100644
--- a/components/style_derive/to_css.rs
+++ b/components/style_derive/to_css.rs
@@ -3,29 +3,26 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use cg;
-use quote;
-use syn;
+use quote::Tokens;
+use syn::DeriveInput;
 use synstructure;
 
-pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
+pub fn derive(input: DeriveInput) -> Tokens {
     let name = &input.ident;
     let trait_path = &["style_traits", "ToCss"];
     let (impl_generics, ty_generics, mut where_clause) =
         cg::trait_parts(&input, trait_path);
 
+    let input_attrs = cg::parse_input_attrs::<CssInputAttrs>(&input);
     let style = synstructure::BindStyle::Ref.into();
     let match_body = synstructure::each_variant(&input, &style, |bindings, variant| {
         let mut identifier = to_css_identifier(variant.ident.as_ref());
-        let css_attrs = cg::parse_variant_attrs::<CssAttrs>(variant);
-        let separator = if css_attrs.comma { ", " } else { " " };
+        let variant_attrs = cg::parse_variant_attrs::<CssVariantAttrs>(variant);
+        let separator = if variant_attrs.comma { ", " } else { " " };
         let mut expr = if !bindings.is_empty() {
             let mut expr = quote! {};
             for binding in bindings {
-                if cg::is_parameterized(&binding.field.ty, &input.generics.ty_params) {
-                    where_clause.predicates.push(
-                        cg::where_predicate(binding.field.ty.clone(), trait_path),
-                    );
-                }
+                where_clause.add_trait_bound(&binding.field.ty);
                 expr = quote! {
                     #expr
                     writer.item(#binding)?;
@@ -41,7 +38,7 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
                 ::std::fmt::Write::write_str(dest, #identifier)
             }
         };
-        if css_attrs.function {
+        if variant_attrs.function {
             identifier.push_str("(");
             expr = quote! {
                 ::std::fmt::Write::write_str(dest, #identifier)?;
@@ -52,7 +49,7 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
         Some(expr)
     });
 
-    quote! {
+    let mut impls = quote! {
         impl #impl_generics ::style_traits::ToCss for #name #ty_generics #where_clause {
             #[allow(unused_variables)]
             #[inline]
@@ -65,12 +62,32 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
                 }
             }
         }
+    };
+
+    if input_attrs.derive_debug {
+        impls.append(quote! {
+            impl #impl_generics ::std::fmt::Debug for #name #ty_generics #where_clause {
+                fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+                    ::style_traits::ToCss::to_css(self, f)
+                }
+            }
+        });
     }
+
+    impls
 }
 
-#[derive(Default, FromVariant)]
 #[darling(attributes(css), default)]
-struct CssAttrs {
+#[derive(Default, FromDeriveInput)]
+struct CssInputAttrs {
+    derive_debug: bool,
+    function: bool,
+    comma: bool,
+}
+
+#[darling(attributes(css), default)]
+#[derive(Default, FromVariant)]
+struct CssVariantAttrs {
     function: bool,
     comma: bool,
 }
diff --git a/components/style_traits/values.rs b/components/style_traits/values.rs
index 5e4c093fd73..94e59e26bbe 100644
--- a/components/style_traits/values.rs
+++ b/components/style_traits/values.rs
@@ -18,8 +18,12 @@ use std::fmt::{self, Write};
 ///   of their name;
 /// * unit variants whose name starts with "Moz" or "Webkit" are prepended
 ///   with a "-";
-/// * variants with fields get serialised as the space-separated serialisations
-///   of their fields.
+/// * if `#[css(comma)]` is found on a variant, its fields are separated by
+///   commas, otherwise, by spaces;
+/// * if `#[css(function)]` is found on a variant, the variant name gets
+///   serialised like unit variants and its fields are surrounded by parentheses;
+/// * finally, one can put `#[css(derive_debug)]` on the whole type, to
+///   implement `Debug` by a single call to `ToCss::to_css`.
 pub trait ToCss {
     /// Serialize `self` in CSS syntax, writing to `dest`.
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: Write;