diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index 10e1f211db3..42204de08f6 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -17,7 +17,7 @@ use values::generics::length::{MaxLength as GenericMaxLength, MozLength as Gener
 use values::generics::NonNegative;
 use values::specified::length::ViewportPercentageLength;
 use values::specified::length::{AbsoluteLength, FontBaseSize, FontRelativeLength};
-use values::{specified, Auto, CSSFloat, Either, Normal};
+use values::{specified, Auto, CSSFloat, Either, Normal, IsAuto};
 
 pub use super::image::Image;
 pub use values::specified::url::UrlOrNone;
@@ -528,6 +528,13 @@ impl LengthOrPercentageOrAuto {
 /// A wrapper of LengthOrPercentageOrAuto, whose value must be >= 0.
 pub type NonNegativeLengthOrPercentageOrAuto = NonNegative<LengthOrPercentageOrAuto>;
 
+impl IsAuto for NonNegativeLengthOrPercentageOrAuto {
+    #[inline]
+    fn is_auto(&self) -> bool {
+        *self == Self::auto()
+    }
+}
+
 impl NonNegativeLengthOrPercentageOrAuto {
     /// `auto`
     #[inline]
diff --git a/components/style/values/generics/background.rs b/components/style/values/generics/background.rs
index a4f4c58d8cd..2f328eaa5e7 100644
--- a/components/style/values/generics/background.rs
+++ b/components/style/values/generics/background.rs
@@ -4,6 +4,10 @@
 
 //! Generic types for CSS values related to backgrounds.
 
+use std::fmt::{self, Write};
+use style_traits::{CssWriter, ToCss};
+use values::IsAuto;
+
 /// A generic value for the `background-size` property.
 #[derive(
     Animate,
@@ -17,7 +21,6 @@
     ToAnimatedValue,
     ToAnimatedZero,
     ToComputedValue,
-    ToCss,
 )]
 pub enum BackgroundSize<LengthOrPercentageOrAuto> {
     /// `<width> <height>`
@@ -34,3 +37,29 @@ pub enum BackgroundSize<LengthOrPercentageOrAuto> {
     #[animation(error)]
     Contain,
 }
+
+impl<LengthOrPercentageOrAuto> ToCss for BackgroundSize<LengthOrPercentageOrAuto>
+where
+    LengthOrPercentageOrAuto: ToCss + IsAuto,
+{
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        match self {
+            BackgroundSize::Explicit { width, height } => {
+                width.to_css(dest)?;
+                // NOTE(emilio): We should probably simplify all these in case
+                // `width == `height`, but all other browsers agree on only
+                // special-casing `auto`.
+                if !width.is_auto() || !height.is_auto() {
+                    dest.write_str(" ")?;
+                    height.to_css(dest)?;
+                }
+                Ok(())
+            }
+            BackgroundSize::Cover => dest.write_str("cover"),
+            BackgroundSize::Contain => dest.write_str("contain"),
+        }
+    }
+}
diff --git a/components/style/values/mod.rs b/components/style/values/mod.rs
index f182477e374..079b579a988 100644
--- a/components/style/values/mod.rs
+++ b/components/style/values/mod.rs
@@ -243,6 +243,13 @@ impl KeyframesName {
 
 impl Eq for KeyframesName {}
 
+/// A trait that returns whether a given type is the `auto` value or not. So far
+/// only needed for background-size serialization, which special-cases `auto`.
+pub trait IsAuto {
+    /// Returns whether the value is the `auto` value.
+    fn is_auto(&self) -> bool;
+}
+
 impl PartialEq for KeyframesName {
     fn eq(&self, other: &Self) -> bool {
         self.as_atom() == other.as_atom()
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 05f5ccd55cd..be5bef27fb8 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -20,7 +20,7 @@ use values::computed::{self, CSSPixelLength, Context, ExtremumLength};
 use values::generics::length::{MaxLength as GenericMaxLength, MozLength as GenericMozLength};
 use values::generics::NonNegative;
 use values::specified::calc::CalcNode;
-use values::{Auto, CSSFloat, Either, Normal};
+use values::{Auto, CSSFloat, Either, Normal, IsAuto};
 
 pub use super::image::{ColorStop, EndingShape as GradientEndingShape, Gradient};
 pub use super::image::{GradientKind, Image};
@@ -980,6 +980,13 @@ impl Parse for LengthOrPercentageOrAuto {
 /// A wrapper of LengthOrPercentageOrAuto, whose value must be >= 0.
 pub type NonNegativeLengthOrPercentageOrAuto = NonNegative<LengthOrPercentageOrAuto>;
 
+impl IsAuto for NonNegativeLengthOrPercentageOrAuto {
+    #[inline]
+    fn is_auto(&self) -> bool {
+        *self == Self::auto()
+    }
+}
+
 impl NonNegativeLengthOrPercentageOrAuto {
     /// 0
     #[inline]