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diff --git a/servo/components/style/values/animated/mod.rs b/servo/components/style/values/animated/mod.rs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Animated values.
+//!
+//! Some values, notably colors, cannot be interpolated directly with their
+//! computed values and need yet another intermediate representation. This
+//! module's raison d'être is to ultimately contain all these types.
+
+use crate::color::AbsoluteColor;
+use crate::properties::PropertyId;
+use crate::values::computed::length::LengthPercentage;
+use crate::values::computed::url::ComputedUrl;
+use crate::values::computed::Angle as ComputedAngle;
+use crate::values::computed::Image;
+use crate::values::specified::SVGPathData;
+use crate::values::CSSFloat;
+use app_units::Au;
+use smallvec::SmallVec;
+use std::cmp;
+
+pub mod color;
+pub mod effects;
+mod font;
+mod grid;
+pub mod lists;
+mod svg;
+pub mod transform;
+
+/// The category a property falls into for ordering purposes.
+///
+/// https://drafts.csswg.org/web-animations/#calculating-computed-keyframes
+#[derive(Clone, Copy, Eq, Ord, PartialEq, PartialOrd)]
+enum PropertyCategory {
+    Custom,
+    PhysicalLonghand,
+    LogicalLonghand,
+    Shorthand,
+}
+
+impl PropertyCategory {
+    fn of(id: &PropertyId) -> Self {
+        match *id {
+            PropertyId::Shorthand(..) | PropertyId::ShorthandAlias(..) => {
+                PropertyCategory::Shorthand
+            },
+            PropertyId::Longhand(id) | PropertyId::LonghandAlias(id, ..) => {
+                if id.is_logical() {
+                    PropertyCategory::LogicalLonghand
+                } else {
+                    PropertyCategory::PhysicalLonghand
+                }
+            },
+            PropertyId::Custom(..) => PropertyCategory::Custom,
+        }
+    }
+}
+
+/// A comparator to sort PropertyIds such that physical longhands are sorted
+/// before logical longhands and shorthands, shorthands with fewer components
+/// are sorted before shorthands with more components, and otherwise shorthands
+/// are sorted by IDL name as defined by [Web Animations][property-order].
+///
+/// Using this allows us to prioritize values specified by longhands (or smaller
+/// shorthand subsets) when longhands and shorthands are both specified on the
+/// one keyframe.
+///
+/// [property-order] https://drafts.csswg.org/web-animations/#calculating-computed-keyframes
+pub fn compare_property_priority(a: &PropertyId, b: &PropertyId) -> cmp::Ordering {
+    let a_category = PropertyCategory::of(a);
+    let b_category = PropertyCategory::of(b);
+
+    if a_category != b_category {
+        return a_category.cmp(&b_category);
+    }
+
+    if a_category != PropertyCategory::Shorthand {
+        return cmp::Ordering::Equal;
+    }
+
+    let a = a.as_shorthand().unwrap();
+    let b = b.as_shorthand().unwrap();
+    // Within shorthands, sort by the number of subproperties, then by IDL
+    // name.
+    let subprop_count_a = a.longhands().count();
+    let subprop_count_b = b.longhands().count();
+    subprop_count_a
+        .cmp(&subprop_count_b)
+        .then_with(|| a.idl_name_sort_order().cmp(&b.idl_name_sort_order()))
+}
+
+/// A helper function to animate two multiplicative factor.
+pub fn animate_multiplicative_factor(
+    this: CSSFloat,
+    other: CSSFloat,
+    procedure: Procedure,
+) -> Result<CSSFloat, ()> {
+    Ok((this - 1.).animate(&(other - 1.), procedure)? + 1.)
+}
+
+/// 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 returns an error.
+///
+/// Trait bounds for type parameter `Foo` can be opted out of with
+/// `#[animation(no_bound(Foo))]` on the type definition, trait bounds for
+/// fields can be opted into with `#[animation(field_bound)]` on the field.
+pub trait Animate: Sized {
+    /// Animate a value towards another one, given an animation procedure.
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()>;
+}
+
+/// An animation procedure.
+///
+/// <https://drafts.csswg.org/web-animations/#procedures-for-animating-properties>
+#[allow(missing_docs)]
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Procedure {
+    /// <https://drafts.csswg.org/web-animations/#animation-interpolation>
+    Interpolate { progress: f64 },
+    /// <https://drafts.csswg.org/web-animations/#animation-addition>
+    Add,
+    /// <https://drafts.csswg.org/web-animations/#animation-accumulation>
+    Accumulate { count: u64 },
+}
+
+/// 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;
+
+    /// Converts this value to an animated value.
+    fn to_animated_value(self) -> Self::AnimatedValue;
+
+    /// Converts back an animated value into a computed value.
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self;
+}
+
+/// 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.
+///
+/// Trait bounds for type parameter `Foo` can be opted out of with
+/// `#[animation(no_bound(Foo))]` on the type definition.
+pub trait ToAnimatedZero: Sized {
+    /// Returns a value that, when added with an underlying value, will produce the underlying
+    /// value. This is used for SMIL animation's "by-animation" where SMIL first interpolates from
+    /// the zero value to the 'by' value, and then adds the result to the underlying value.
+    ///
+    /// This is not the necessarily the same as the initial value of a property. For example, the
+    /// initial value of 'stroke-width' is 1, but the zero value is 0, since adding 1 to the
+    /// underlying value will not produce the underlying value.
+    fn to_animated_zero(&self) -> Result<Self, ()>;
+}
+
+impl Procedure {
+    /// Returns this procedure as a pair of weights.
+    ///
+    /// This is useful for animations that don't animate differently
+    /// depending on the used procedure.
+    #[inline]
+    pub fn weights(self) -> (f64, f64) {
+        match self {
+            Procedure::Interpolate { progress } => (1. - progress, progress),
+            Procedure::Add => (1., 1.),
+            Procedure::Accumulate { count } => (count as f64, 1.),
+        }
+    }
+}
+
+/// <https://drafts.csswg.org/css-transitions/#animtype-number>
+impl Animate for i32 {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        Ok(((*self as f64).animate(&(*other as f64), procedure)? + 0.5).floor() as i32)
+    }
+}
+
+/// <https://drafts.csswg.org/css-transitions/#animtype-number>
+impl Animate for f32 {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        let ret = (*self as f64).animate(&(*other as f64), procedure)?;
+        Ok(ret.min(f32::MAX as f64).max(f32::MIN as f64) as f32)
+    }
+}
+
+/// <https://drafts.csswg.org/css-transitions/#animtype-number>
+impl Animate for f64 {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        let (self_weight, other_weight) = procedure.weights();
+
+        let ret = *self * self_weight + *other * other_weight;
+        Ok(ret.min(f64::MAX).max(f64::MIN))
+    }
+}
+
+impl<T> Animate for Option<T>
+where
+    T: Animate,
+{
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        match (self.as_ref(), other.as_ref()) {
+            (Some(ref this), Some(ref other)) => Ok(Some(this.animate(other, procedure)?)),
+            (None, None) => Ok(None),
+            _ => Err(()),
+        }
+    }
+}
+
+impl Animate for Au {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        Ok(Au::new(self.0.animate(&other.0, procedure)?))
+    }
+}
+
+impl<T: Animate> Animate for Box<T> {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        Ok(Box::new((**self).animate(&other, procedure)?))
+    }
+}
+
+impl<T> ToAnimatedValue for Option<T>
+where
+    T: ToAnimatedValue,
+{
+    type AnimatedValue = Option<<T as ToAnimatedValue>::AnimatedValue>;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        self.map(T::to_animated_value)
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        animated.map(T::from_animated_value)
+    }
+}
+
+impl<T> ToAnimatedValue for Vec<T>
+where
+    T: ToAnimatedValue,
+{
+    type AnimatedValue = Vec<<T as ToAnimatedValue>::AnimatedValue>;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        self.into_iter().map(T::to_animated_value).collect()
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        animated.into_iter().map(T::from_animated_value).collect()
+    }
+}
+
+impl<T> ToAnimatedValue for Box<T>
+where
+    T: ToAnimatedValue,
+{
+    type AnimatedValue = Box<<T as ToAnimatedValue>::AnimatedValue>;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        Box::new((*self).to_animated_value())
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        Box::new(T::from_animated_value(*animated))
+    }
+}
+
+impl<T> ToAnimatedValue for Box<[T]>
+where
+    T: ToAnimatedValue,
+{
+    type AnimatedValue = Box<[<T as ToAnimatedValue>::AnimatedValue]>;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        self.into_vec()
+            .into_iter()
+            .map(T::to_animated_value)
+            .collect::<Vec<_>>()
+            .into_boxed_slice()
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        animated
+            .into_vec()
+            .into_iter()
+            .map(T::from_animated_value)
+            .collect::<Vec<_>>()
+            .into_boxed_slice()
+    }
+}
+
+impl<T> ToAnimatedValue for crate::OwnedSlice<T>
+where
+    T: ToAnimatedValue,
+{
+    type AnimatedValue = crate::OwnedSlice<<T as ToAnimatedValue>::AnimatedValue>;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        self.into_box().to_animated_value().into()
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        Self::from(Box::from_animated_value(animated.into_box()))
+    }
+}
+
+impl<T> ToAnimatedValue for SmallVec<[T; 1]>
+where
+    T: ToAnimatedValue,
+{
+    type AnimatedValue = SmallVec<[T::AnimatedValue; 1]>;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        self.into_iter().map(T::to_animated_value).collect()
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        animated.into_iter().map(T::from_animated_value).collect()
+    }
+}
+
+macro_rules! trivial_to_animated_value {
+    ($ty:ty) => {
+        impl $crate::values::animated::ToAnimatedValue for $ty {
+            type AnimatedValue = Self;
+
+            #[inline]
+            fn to_animated_value(self) -> Self {
+                self
+            }
+
+            #[inline]
+            fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+                animated
+            }
+        }
+    };
+}
+
+trivial_to_animated_value!(Au);
+trivial_to_animated_value!(LengthPercentage);
+trivial_to_animated_value!(ComputedAngle);
+trivial_to_animated_value!(ComputedUrl);
+trivial_to_animated_value!(bool);
+trivial_to_animated_value!(f32);
+trivial_to_animated_value!(i32);
+trivial_to_animated_value!(AbsoluteColor);
+// Note: This implementation is for ToAnimatedValue of ShapeSource.
+//
+// SVGPathData uses Box<[T]>. If we want to derive ToAnimatedValue for all the
+// types, we have to do "impl ToAnimatedValue for Box<[T]>" first.
+// However, the general version of "impl ToAnimatedValue for Box<[T]>" needs to
+// clone |T| and convert it into |T::AnimatedValue|. However, for SVGPathData
+// that is unnecessary--moving |T| is sufficient. So here, we implement this
+// trait manually.
+trivial_to_animated_value!(SVGPathData);
+// FIXME: Bug 1514342, Image is not animatable, but we still need to implement
+// this to avoid adding this derive to generic::Image and all its arms. We can
+// drop this after landing Bug 1514342.
+trivial_to_animated_value!(Image);
+
+impl ToAnimatedZero for Au {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(Au(0))
+    }
+}
+
+impl ToAnimatedZero for f32 {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(0.)
+    }
+}
+
+impl ToAnimatedZero for f64 {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(0.)
+    }
+}
+
+impl ToAnimatedZero for i32 {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(0)
+    }
+}
+
+impl<T> ToAnimatedZero for Box<T>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(Box::new((**self).to_animated_zero()?))
+    }
+}
+
+impl<T> ToAnimatedZero for Option<T>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        match *self {
+            Some(ref value) => Ok(Some(value.to_animated_zero()?)),
+            None => Ok(None),
+        }
+    }
+}
+
+impl<T> ToAnimatedZero for Vec<T>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        self.iter().map(|v| v.to_animated_zero()).collect()
+    }
+}
+
+impl<T> ToAnimatedZero for Box<[T]>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        self.iter().map(|v| v.to_animated_zero()).collect()
+    }
+}
+
+impl<T> ToAnimatedZero for crate::OwnedSlice<T>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        self.iter().map(|v| v.to_animated_zero()).collect()
+    }
+}
+
+impl<T> ToAnimatedZero for crate::ArcSlice<T>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        let v = self
+            .iter()
+            .map(|v| v.to_animated_zero())
+            .collect::<Result<Vec<_>, _>>()?;
+        Ok(crate::ArcSlice::from_iter(v.into_iter()))
+    }
+}