diff --git a/components/style/values/computed/page.rs b/components/style/values/computed/page.rs
index d8ef03c1f6d..524f6ba468e 100644
--- a/components/style/values/computed/page.rs
+++ b/components/style/values/computed/page.rs
@@ -5,10 +5,71 @@
 //! Computed @page at-rule properties
 
 use crate::values::computed::length::NonNegativeLength;
+use crate::values::computed::{Context, ToComputedValue};
 use crate::values::generics;
 use crate::values::generics::size::Size2D;
 
+use crate::values::specified::page as specified;
+pub use generics::page::GenericPageSize;
 pub use generics::page::Orientation;
 pub use generics::page::PaperSize;
+
 /// Computed value of the @page size descriptor
-pub type PageSize = generics::page::GenericPageSize<Size2D<NonNegativeLength>>;
+///
+/// The spec says that the computed value should be the same as the specified
+/// value but with all absolute units, but it's not currently possibly observe
+/// the computed value of page-size.
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, ToCss, ToResolvedValue, ToShmem)]
+#[repr(C, u8)]
+pub enum PageSize {
+    /// Specified size, paper size, or paper size and orientation.
+    Size(Size2D<NonNegativeLength>),
+    /// `landscape` or `portrait` value, no specified size.
+    Orientation(Orientation),
+    /// `auto` value
+    Auto,
+}
+
+impl ToComputedValue for specified::PageSize {
+    type ComputedValue = PageSize;
+
+    fn to_computed_value(&self, ctx: &Context) -> Self::ComputedValue {
+        match &*self {
+            Self::Size(s) => PageSize::Size(s.to_computed_value(ctx)),
+            Self::PaperSizeAndOrientation(p, Orientation::Landscape) => PageSize::Size(Size2D {
+                width: p.long_edge().to_computed_value(ctx),
+                height: p.short_edge().to_computed_value(ctx),
+            }),
+            Self::PaperSizeAndOrientation(p, Orientation::Portrait) | Self::PaperSize(p) => {
+                PageSize::Size(Size2D {
+                    width: p.short_edge().to_computed_value(ctx),
+                    height: p.long_edge().to_computed_value(ctx),
+                })
+            },
+            Self::Orientation(o) => PageSize::Orientation(*o),
+            Self::Auto => PageSize::Auto,
+        }
+    }
+
+    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
+        match *computed {
+            PageSize::Size(s) => Self::Size(ToComputedValue::from_computed_value(&s)),
+            PageSize::Orientation(o) => Self::Orientation(o),
+            PageSize::Auto => Self::Auto,
+        }
+    }
+}
+
+impl PageSize {
+    /// `auto` value.
+    #[inline]
+    pub fn auto() -> Self {
+        PageSize::Auto
+    }
+
+    /// Whether this is the `auto` value.
+    #[inline]
+    pub fn is_auto(&self) -> bool {
+        matches!(*self, PageSize::Auto)
+    }
+}
diff --git a/components/style/values/generics/page.rs b/components/style/values/generics/page.rs
index 35ffee7b97e..6f561710491 100644
--- a/components/style/values/generics/page.rs
+++ b/components/style/values/generics/page.rs
@@ -4,22 +4,14 @@
 
 //! @page at-rule properties
 
+use crate::values::generics::NonNegative;
+use crate::values::specified::length::AbsoluteLength;
+
 /// Page size names.
 ///
 /// https://drafts.csswg.org/css-page-3/#typedef-page-size-page-size
 #[derive(
-    Clone,
-    Copy,
-    Debug,
-    Eq,
-    MallocSizeOf,
-    Parse,
-    PartialEq,
-    SpecifiedValueInfo,
-    ToComputedValue,
-    ToCss,
-    ToResolvedValue,
-    ToShmem,
+    Clone, Copy, Debug, Eq, MallocSizeOf, Parse, PartialEq, SpecifiedValueInfo, ToCss, ToShmem,
 )]
 #[repr(u8)]
 pub enum PaperSize {
@@ -45,6 +37,39 @@ pub enum PaperSize {
     Ledger,
 }
 
+impl PaperSize {
+    /// Gets the long edge length of the paper size
+    pub fn long_edge(&self) -> NonNegative<AbsoluteLength> {
+        NonNegative(match *self {
+            PaperSize::A5 => AbsoluteLength::Mm(210.0),
+            PaperSize::A4 => AbsoluteLength::Mm(297.0),
+            PaperSize::A3 => AbsoluteLength::Mm(420.0),
+            PaperSize::B5 => AbsoluteLength::Mm(250.0),
+            PaperSize::B4 => AbsoluteLength::Mm(353.0),
+            PaperSize::JisB5 => AbsoluteLength::Mm(257.0),
+            PaperSize::JisB4 => AbsoluteLength::Mm(364.0),
+            PaperSize::Letter => AbsoluteLength::In(11.0),
+            PaperSize::Legal => AbsoluteLength::In(14.0),
+            PaperSize::Ledger => AbsoluteLength::In(17.0),
+        })
+    }
+    /// Gets the short edge length of the paper size
+    pub fn short_edge(&self) -> NonNegative<AbsoluteLength> {
+        NonNegative(match *self {
+            PaperSize::A5 => AbsoluteLength::Mm(148.0),
+            PaperSize::A4 => AbsoluteLength::Mm(210.0),
+            PaperSize::A3 => AbsoluteLength::Mm(297.0),
+            PaperSize::B5 => AbsoluteLength::Mm(176.0),
+            PaperSize::B4 => AbsoluteLength::Mm(250.0),
+            PaperSize::JisB5 => AbsoluteLength::Mm(182.0),
+            PaperSize::JisB4 => AbsoluteLength::Mm(257.0),
+            PaperSize::Letter => AbsoluteLength::In(8.5),
+            PaperSize::Legal => AbsoluteLength::In(8.5),
+            PaperSize::Ledger => AbsoluteLength::In(11.0),
+        })
+    }
+}
+
 /// Paper orientation
 ///
 /// https://drafts.csswg.org/css-page-3/#page-size-prop
@@ -57,7 +82,6 @@ pub enum PaperSize {
     Parse,
     PartialEq,
     SpecifiedValueInfo,
-    ToComputedValue,
     ToCss,
     ToResolvedValue,
     ToShmem,
@@ -73,18 +97,7 @@ pub enum Orientation {
 /// Page size property
 ///
 /// https://drafts.csswg.org/css-page-3/#page-size-prop
-#[derive(
-    Clone,
-    Copy,
-    Debug,
-    MallocSizeOf,
-    PartialEq,
-    SpecifiedValueInfo,
-    ToComputedValue,
-    ToCss,
-    ToResolvedValue,
-    ToShmem,
-)]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToCss, ToShmem)]
 #[repr(C, u8)]
 pub enum GenericPageSize<S> {
     /// Page dimensions.