diff --git a/components/layout_2020/lib.rs b/components/layout_2020/lib.rs
index 09013578308..dd589e1819b 100644
--- a/components/layout_2020/lib.rs
+++ b/components/layout_2020/lib.rs
@@ -4,6 +4,7 @@
 
 #![deny(unsafe_code)]
 #![feature(exact_size_is_empty)]
+#![feature(matches_macro)]
 
 pub mod context;
 pub mod data;
diff --git a/components/layout_2020/style_ext.rs b/components/layout_2020/style_ext.rs
index e6d4537f47c..6bd2e91a2f9 100644
--- a/components/layout_2020/style_ext.rs
+++ b/components/layout_2020/style_ext.rs
@@ -43,6 +43,8 @@ pub(crate) enum DisplayInside {
 
 pub(crate) trait ComputedValuesExt {
     fn writing_mode(&self) -> (WritingMode, Direction);
+    fn writing_mode_is_horizontal(&self) -> bool;
+    fn inline_size_is_auto(&self) -> bool;
     fn box_offsets(&self) -> flow_relative::Sides<LengthPercentageOrAuto>;
     fn box_size(&self) -> flow_relative::Vec2<LengthPercentageOrAuto>;
     fn padding(&self) -> flow_relative::Sides<LengthPercentage>;
@@ -58,6 +60,23 @@ impl ComputedValuesExt for ComputedValues {
         (writing_mode, direction)
     }
 
+    fn writing_mode_is_horizontal(&self) -> bool {
+        match self.get_inherited_box().writing_mode {
+            WritingMode::HorizontalTb => true,
+            WritingMode::VerticalLr | WritingMode::VerticalRl => false,
+        }
+    }
+
+    fn inline_size_is_auto(&self) -> bool {
+        let position = self.get_position();
+        let size = if self.writing_mode_is_horizontal() {
+            position.width
+        } else {
+            position.height
+        };
+        matches!(size, Size::Auto)
+    }
+
     #[inline]
     fn box_offsets(&self) -> flow_relative::Sides<LengthPercentageOrAuto> {
         let position = self.get_position();