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diff --git a/servo/components/style/values/generics/length.rs b/servo/components/style/values/generics/length.rs
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+++ b/servo/components/style/values/generics/length.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/. */
+
+//! Generic types for CSS values related to length.
+
+use crate::parser::{Parse, ParserContext};
+#[cfg(feature = "gecko")]
+use crate::Zero;
+use cssparser::Parser;
+use style_traits::ParseError;
+
+/// A `<length-percentage> | auto` value.
+#[allow(missing_docs)]
+#[derive(
+    Animate,
+    Clone,
+    ComputeSquaredDistance,
+    Copy,
+    Debug,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToAnimatedValue,
+    ToAnimatedZero,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum GenericLengthPercentageOrAuto<LengthPercent> {
+    LengthPercentage(LengthPercent),
+    Auto,
+}
+
+pub use self::GenericLengthPercentageOrAuto as LengthPercentageOrAuto;
+
+impl<LengthPercentage> LengthPercentageOrAuto<LengthPercentage> {
+    /// `auto` value.
+    #[inline]
+    pub fn auto() -> Self {
+        LengthPercentageOrAuto::Auto
+    }
+
+    /// Whether this is the `auto` value.
+    #[inline]
+    pub fn is_auto(&self) -> bool {
+        matches!(*self, LengthPercentageOrAuto::Auto)
+    }
+
+    /// A helper function to parse this with quirks or not and so forth.
+    pub fn parse_with<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+        parser: impl FnOnce(
+            &ParserContext,
+            &mut Parser<'i, 't>,
+        ) -> Result<LengthPercentage, ParseError<'i>>,
+    ) -> Result<Self, ParseError<'i>> {
+        if input.try_parse(|i| i.expect_ident_matching("auto")).is_ok() {
+            return Ok(LengthPercentageOrAuto::Auto);
+        }
+
+        Ok(LengthPercentageOrAuto::LengthPercentage(parser(
+            context, input,
+        )?))
+    }
+}
+
+impl<LengthPercentage> LengthPercentageOrAuto<LengthPercentage>
+where
+    LengthPercentage: Clone,
+{
+    /// Resolves `auto` values by calling `f`.
+    #[inline]
+    pub fn auto_is(&self, f: impl FnOnce() -> LengthPercentage) -> LengthPercentage {
+        match self {
+            LengthPercentageOrAuto::LengthPercentage(length) => length.clone(),
+            LengthPercentageOrAuto::Auto => f(),
+        }
+    }
+
+    /// Returns the non-`auto` value, if any.
+    #[inline]
+    pub fn non_auto(&self) -> Option<LengthPercentage> {
+        match self {
+            LengthPercentageOrAuto::LengthPercentage(length) => Some(length.clone()),
+            LengthPercentageOrAuto::Auto => None,
+        }
+    }
+
+    /// Maps the length of this value.
+    pub fn map<T>(&self, f: impl FnOnce(LengthPercentage) -> T) -> LengthPercentageOrAuto<T> {
+        match self {
+            LengthPercentageOrAuto::LengthPercentage(l) => {
+                LengthPercentageOrAuto::LengthPercentage(f(l.clone()))
+            },
+            LengthPercentageOrAuto::Auto => LengthPercentageOrAuto::Auto,
+        }
+    }
+}
+
+impl<LengthPercentage: Zero> Zero for LengthPercentageOrAuto<LengthPercentage> {
+    fn zero() -> Self {
+        LengthPercentageOrAuto::LengthPercentage(Zero::zero())
+    }
+
+    fn is_zero(&self) -> bool {
+        match *self {
+            LengthPercentageOrAuto::LengthPercentage(ref l) => l.is_zero(),
+            LengthPercentageOrAuto::Auto => false,
+        }
+    }
+}
+
+impl<LengthPercentage: Parse> Parse for LengthPercentageOrAuto<LengthPercentage> {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        Self::parse_with(context, input, LengthPercentage::parse)
+    }
+}
+
+/// A generic value for the `width`, `height`, `min-width`, or `min-height` property.
+///
+/// Unlike `max-width` or `max-height` properties, a Size can be `auto`,
+/// and cannot be `none`.
+///
+/// Note that it only accepts non-negative values.
+#[allow(missing_docs)]
+#[derive(
+    Animate,
+    Clone,
+    ComputeSquaredDistance,
+    Copy,
+    Debug,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToAnimatedValue,
+    ToAnimatedZero,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum GenericSize<LengthPercent> {
+    LengthPercentage(LengthPercent),
+    Auto,
+    #[animation(error)]
+    MaxContent,
+    #[animation(error)]
+    MinContent,
+    #[animation(error)]
+    FitContent,
+    #[animation(error)]
+    MozAvailable,
+    #[animation(error)]
+    #[css(function = "fit-content")]
+    FitContentFunction(LengthPercent),
+}
+
+pub use self::GenericSize as Size;
+
+impl<LengthPercentage> Size<LengthPercentage> {
+    /// `auto` value.
+    #[inline]
+    pub fn auto() -> Self {
+        Size::Auto
+    }
+
+    /// Returns whether we're the auto value.
+    #[inline]
+    pub fn is_auto(&self) -> bool {
+        matches!(*self, Size::Auto)
+    }
+}
+
+/// A generic value for the `max-width` or `max-height` property.
+#[allow(missing_docs)]
+#[derive(
+    Animate,
+    Clone,
+    ComputeSquaredDistance,
+    Copy,
+    Debug,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToAnimatedValue,
+    ToAnimatedZero,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum GenericMaxSize<LengthPercent> {
+    LengthPercentage(LengthPercent),
+    None,
+    #[animation(error)]
+    MaxContent,
+    #[animation(error)]
+    MinContent,
+    #[animation(error)]
+    FitContent,
+    #[animation(error)]
+    MozAvailable,
+    #[animation(error)]
+    #[css(function = "fit-content")]
+    FitContentFunction(LengthPercent),
+}
+
+pub use self::GenericMaxSize as MaxSize;
+
+impl<LengthPercentage> MaxSize<LengthPercentage> {
+    /// `none` value.
+    #[inline]
+    pub fn none() -> Self {
+        MaxSize::None
+    }
+}
+
+/// A generic `<length>` | `<number>` value for the `tab-size` property.
+#[derive(
+    Animate,
+    Clone,
+    ComputeSquaredDistance,
+    Copy,
+    Debug,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToAnimatedValue,
+    ToAnimatedZero,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum GenericLengthOrNumber<L, N> {
+    /// A number.
+    ///
+    /// NOTE: Numbers need to be before lengths, in order to parse them
+    /// first, since `0` should be a number, not the `0px` length.
+    Number(N),
+    /// A length.
+    Length(L),
+}
+
+pub use self::GenericLengthOrNumber as LengthOrNumber;
+
+impl<L, N: Zero> Zero for LengthOrNumber<L, N> {
+    fn zero() -> Self {
+        LengthOrNumber::Number(Zero::zero())
+    }
+
+    fn is_zero(&self) -> bool {
+        match *self {
+            LengthOrNumber::Number(ref n) => n.is_zero(),
+            LengthOrNumber::Length(..) => false,
+        }
+    }
+}
+
+/// A generic `<length-percentage>` | normal` value.
+#[derive(
+    Animate,
+    Clone,
+    ComputeSquaredDistance,
+    Copy,
+    Debug,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToAnimatedValue,
+    ToAnimatedZero,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+#[allow(missing_docs)]
+pub enum GenericLengthPercentageOrNormal<LengthPercent> {
+    LengthPercentage(LengthPercent),
+    Normal,
+}
+
+pub use self::GenericLengthPercentageOrNormal as LengthPercentageOrNormal;
+
+impl<LengthPercent> LengthPercentageOrNormal<LengthPercent> {
+    /// Returns the normal value.
+    #[inline]
+    pub fn normal() -> Self {
+        LengthPercentageOrNormal::Normal
+    }
+}