diff --git a/Meta/gn/secondary/Userland/Libraries/LibWeb/CSS/BUILD.gn b/Meta/gn/secondary/Userland/Libraries/LibWeb/CSS/BUILD.gn
index b46d00d4d9e..780c425244d 100644
--- a/Meta/gn/secondary/Userland/Libraries/LibWeb/CSS/BUILD.gn
+++ b/Meta/gn/secondary/Userland/Libraries/LibWeb/CSS/BUILD.gn
@@ -60,6 +60,7 @@ source_set("CSS") {
     "SelectorEngine.cpp",
     "Serialize.cpp",
     "Size.cpp",
+    "Sizing.cpp",
     "StyleComputer.cpp",
     "StyleInvalidation.cpp",
     "StyleProperties.cpp",
diff --git a/Userland/Libraries/LibWeb/CMakeLists.txt b/Userland/Libraries/LibWeb/CMakeLists.txt
index c38d2f065e5..883138bba27 100644
--- a/Userland/Libraries/LibWeb/CMakeLists.txt
+++ b/Userland/Libraries/LibWeb/CMakeLists.txt
@@ -99,6 +99,7 @@ set(SOURCES
     CSS/SelectorEngine.cpp
     CSS/Serialize.cpp
     CSS/Size.cpp
+    CSS/Sizing.cpp
     CSS/StyleComputer.cpp
     CSS/StyleInvalidation.cpp
     CSS/StyleProperties.cpp
diff --git a/Userland/Libraries/LibWeb/CSS/Sizing.cpp b/Userland/Libraries/LibWeb/CSS/Sizing.cpp
new file mode 100644
index 00000000000..74d58c7c8a9
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/Sizing.cpp
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2022, MacDue <macdue@dueutil.tech>
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Sizing.h"
+
+namespace Web::CSS {
+
+// https://drafts.csswg.org/css-images/#default-sizing
+CSSPixelSize run_default_sizing_algorithm(
+    Optional<CSSPixels> specified_width, Optional<CSSPixels> specified_height,
+    Optional<CSSPixels> natural_width, Optional<CSSPixels> natural_height,
+    Optional<CSSPixelFraction> natural_aspect_ratio,
+    CSSPixelSize default_size)
+{
+    // If the specified size is a definite width and height, the concrete object size is given that width and height.
+    if (specified_width.has_value() && specified_height.has_value())
+        return CSSPixelSize { specified_width.value(), specified_height.value() };
+    // If the specified size is only a width or height (but not both) then the concrete object size is given that specified width or height.
+    // The other dimension is calculated as follows:
+    if (specified_width.has_value() || specified_height.has_value()) {
+        // 1. If the object has a natural aspect ratio,
+        // the missing dimension of the concrete object size is calculated using that aspect ratio and the present dimension.
+        if (natural_aspect_ratio.has_value() && !natural_aspect_ratio->might_be_saturated()) {
+            if (specified_width.has_value())
+                return CSSPixelSize { specified_width.value(), (CSSPixels(1) / natural_aspect_ratio.value()) * specified_width.value() };
+            if (specified_height.has_value())
+                return CSSPixelSize { specified_height.value() * natural_aspect_ratio.value(), specified_height.value() };
+        }
+        // 2. Otherwise, if the missing dimension is present in the object’s natural dimensions,
+        // the missing dimension is taken from the object’s natural dimensions.
+        if (specified_height.has_value() && natural_width.has_value())
+            return CSSPixelSize { natural_width.value(), specified_height.value() };
+        if (specified_width.has_value() && natural_height.has_value())
+            return CSSPixelSize { specified_width.value(), natural_height.value() };
+        // 3. Otherwise, the missing dimension of the concrete object size is taken from the default object size.
+        if (specified_height.has_value())
+            return CSSPixelSize { default_size.width(), specified_height.value() };
+        if (specified_width.has_value())
+            return CSSPixelSize { specified_width.value(), default_size.height() };
+        VERIFY_NOT_REACHED();
+    }
+    // If the specified size has no constraints:
+    // 1. If the object has a natural height or width, its size is resolved as if its natural dimensions were given as the specified size.
+    if (natural_width.has_value() || natural_height.has_value())
+        return run_default_sizing_algorithm(natural_width, natural_height, natural_width, natural_height, natural_aspect_ratio, default_size);
+    // FIXME: 2. Otherwise, its size is resolved as a contain constraint against the default object size.
+    return default_size;
+}
+
+}
diff --git a/Userland/Libraries/LibWeb/CSS/Sizing.h b/Userland/Libraries/LibWeb/CSS/Sizing.h
new file mode 100644
index 00000000000..f1cc8e006b3
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/Sizing.h
@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/PixelUnits.h>
+
+namespace Web::CSS {
+
+// https://drafts.csswg.org/css-images/#default-sizing
+CSSPixelSize run_default_sizing_algorithm(
+    Optional<CSSPixels> specified_width, Optional<CSSPixels> specified_height,
+    Optional<CSSPixels> natural_width, Optional<CSSPixels> natural_height,
+    Optional<CSSPixelFraction> natural_aspect_ratio,
+    CSSPixelSize default_size);
+
+}
diff --git a/Userland/Libraries/LibWeb/Painting/BackgroundPainting.cpp b/Userland/Libraries/LibWeb/Painting/BackgroundPainting.cpp
index 920f23cc6f9..324339c537b 100644
--- a/Userland/Libraries/LibWeb/Painting/BackgroundPainting.cpp
+++ b/Userland/Libraries/LibWeb/Painting/BackgroundPainting.cpp
@@ -6,6 +6,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/CSS/Sizing.h>
 #include <LibWeb/Layout/Node.h>
 #include <LibWeb/Layout/Viewport.h>
 #include <LibWeb/Painting/BackgroundPainting.h>
@@ -14,48 +15,6 @@
 
 namespace Web::Painting {
 
-// https://drafts.csswg.org/css-images/#default-sizing
-static CSSPixelSize run_default_sizing_algorithm(
-    Optional<CSSPixels> specified_width, Optional<CSSPixels> specified_height,
-    Optional<CSSPixels> natural_width, Optional<CSSPixels> natural_height,
-    Optional<CSSPixelFraction> natural_aspect_ratio,
-    CSSPixelSize default_size)
-{
-    // If the specified size is a definite width and height, the concrete object size is given that width and height.
-    if (specified_width.has_value() && specified_height.has_value())
-        return CSSPixelSize { specified_width.value(), specified_height.value() };
-    // If the specified size is only a width or height (but not both) then the concrete object size is given that specified width or height.
-    // The other dimension is calculated as follows:
-    if (specified_width.has_value() || specified_height.has_value()) {
-        // 1. If the object has a natural aspect ratio,
-        // the missing dimension of the concrete object size is calculated using that aspect ratio and the present dimension.
-        if (natural_aspect_ratio.has_value() && !natural_aspect_ratio->might_be_saturated()) {
-            if (specified_width.has_value())
-                return CSSPixelSize { specified_width.value(), (CSSPixels(1) / natural_aspect_ratio.value()) * specified_width.value() };
-            if (specified_height.has_value())
-                return CSSPixelSize { specified_height.value() * natural_aspect_ratio.value(), specified_height.value() };
-        }
-        // 2. Otherwise, if the missing dimension is present in the object’s natural dimensions,
-        // the missing dimension is taken from the object’s natural dimensions.
-        if (specified_height.has_value() && natural_width.has_value())
-            return CSSPixelSize { natural_width.value(), specified_height.value() };
-        if (specified_width.has_value() && natural_height.has_value())
-            return CSSPixelSize { specified_width.value(), natural_height.value() };
-        // 3. Otherwise, the missing dimension of the concrete object size is taken from the default object size.
-        if (specified_height.has_value())
-            return CSSPixelSize { default_size.width(), specified_height.value() };
-        if (specified_width.has_value())
-            return CSSPixelSize { specified_width.value(), default_size.height() };
-        VERIFY_NOT_REACHED();
-    }
-    // If the specified size has no constraints:
-    // 1. If the object has a natural height or width, its size is resolved as if its natural dimensions were given as the specified size.
-    if (natural_width.has_value() || natural_height.has_value())
-        return run_default_sizing_algorithm(natural_width, natural_height, natural_width, natural_height, natural_aspect_ratio, default_size);
-    // FIXME: 2. Otherwise, its size is resolved as a contain constraint against the default object size.
-    return default_size;
-}
-
 static RefPtr<DisplayList> compute_text_clip_paths(PaintContext& context, Paintable const& paintable, CSSPixelPoint containing_block_location)
 {
     auto text_clip_paths = DisplayList::create();
@@ -373,7 +332,7 @@ ResolvedBackground resolve_background_layers(Vector<CSS::BackgroundLayerData> co
             if (!layer.size_y.is_auto())
                 specified_height = layer.size_y.to_px(layout_node, background_positioning_area.height());
         }
-        auto concrete_image_size = run_default_sizing_algorithm(
+        auto concrete_image_size = CSS::run_default_sizing_algorithm(
             specified_width, specified_height,
             image.natural_width(), image.natural_height(), image.natural_aspect_ratio(),
             background_positioning_area.size());