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diff --git a/layout/generic/AspectRatio.h b/layout/generic/AspectRatio.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_AspectRatio_h
+#define mozilla_AspectRatio_h
+
+/* The aspect ratio of a box, in a "width / height" format. */
+
+#include "mozilla/Attributes.h"
+#include "mozilla/gfx/BaseSize.h"
+#include "nsCoord.h"
+#include <algorithm>
+#include <limits>
+
+namespace IPC {
+template <typename T>
+struct ParamTraits;
+}  // namespace IPC
+
+namespace mozilla {
+
+enum LogicalAxis : uint8_t;
+class LogicalSize;
+class WritingMode;
+
+enum class UseBoxSizing : uint8_t {
+  // The aspect ratio works with content box dimensions always.
+  No,
+  // The aspect ratio works with the dimensions of the box specified by
+  // box-sizing.
+  Yes,
+};
+
+struct AspectRatio {
+  friend struct IPC::ParamTraits<mozilla::AspectRatio>;
+
+  AspectRatio() = default;
+  explicit AspectRatio(float aRatio,
+                       UseBoxSizing aUseBoxSizing = UseBoxSizing::No)
+      : mRatio(std::max(aRatio, 0.0f)), mUseBoxSizing(aUseBoxSizing) {}
+
+  static AspectRatio FromSize(float aWidth, float aHeight,
+                              UseBoxSizing aUseBoxSizing = UseBoxSizing::No) {
+    if (aWidth == 0.0f || aHeight == 0.0f) {
+      // For the degenerate ratio, we don't care about which box sizing we are
+      // using, so using default constructor is fine.
+      return AspectRatio();
+    }
+    return AspectRatio(aWidth / aHeight, aUseBoxSizing);
+  }
+
+  template <typename T, typename Sub>
+  static AspectRatio FromSize(const gfx::BaseSize<T, Sub>& aSize) {
+    return FromSize(aSize.Width(), aSize.Height());
+  }
+
+  explicit operator bool() const { return mRatio != 0.0f; }
+
+  nscoord ApplyTo(nscoord aCoord) const {
+    MOZ_DIAGNOSTIC_ASSERT(*this);
+    return NSCoordSaturatingNonnegativeMultiply(aCoord, mRatio);
+  }
+
+  float ApplyToFloat(float aFloat) const {
+    MOZ_DIAGNOSTIC_ASSERT(*this);
+    return mRatio * aFloat;
+  }
+
+  // Inverts the ratio, in order to get the height / width ratio.
+  [[nodiscard]] AspectRatio Inverted() const {
+    if (!*this) {
+      return AspectRatio();
+    }
+    // Clamp to a small epsilon, in case mRatio is absurdly large & produces
+    // 0.0f in the division here (so that valid ratios always generate other
+    // valid ratios when inverted).
+    return AspectRatio(
+        std::max(std::numeric_limits<float>::epsilon(), 1.0f / mRatio),
+        mUseBoxSizing);
+  }
+
+  [[nodiscard]] inline AspectRatio ConvertToWritingMode(
+      const WritingMode& aWM) const;
+
+  /**
+   * This method computes the ratio-dependent size by the ratio-determining size
+   * and aspect-ratio (i.e. preferred aspect ratio). Basically this function
+   * will be used in the calculation of 'auto' sizes when the preferred
+   * aspect ratio is not 'auto'.
+   *
+   * @param aRatioDependentAxis  The ratio depenedent axis of the box.
+   * @param aWM  The writing mode of the box.
+   * @param aRatioDetermingSize  The content-box size on the ratio determining
+   *                             axis. Basically, we use this size and |mRatio|
+   *                             to compute the size on the ratio-dependent
+   *                             axis.
+   * @param aContentBoxSizeToBoxSizingAdjust  The border padding box size
+   *                                          adjustment. We need this because
+   *                                          aspect-ratio should take the
+   *                                          box-sizing into account if its
+   *                                          style is '<ratio>'. If its style
+   *                                          is 'auto & <ratio>', we should use
+   *                                          content-box dimensions always.
+   *                                          If the callers want the ratio to
+   *                                          apply to the content-box size, we
+   *                                          should pass a zero LogicalSize.
+   *                                          If mUseBoxSizing is No, we ignore
+   *                                          this parameter because we should
+   *                                          use content box dimensions always.
+   *
+   * The return value is the content-box size on the ratio-dependent axis.
+   * Plese see the definition of the ratio-dependent axis and the
+   * ratio-determining axis in the spec:
+   * https://drafts.csswg.org/css-sizing-4/#aspect-ratio
+   */
+  [[nodiscard]] nscoord ComputeRatioDependentSize(
+      LogicalAxis aRatioDependentAxis, const WritingMode& aWM,
+      nscoord aRatioDeterminingSize,
+      const LogicalSize& aContentBoxSizeToBoxSizingAdjust) const;
+
+  bool operator==(const AspectRatio& aOther) const {
+    return mRatio == aOther.mRatio && mUseBoxSizing == aOther.mUseBoxSizing;
+  }
+
+  bool operator!=(const AspectRatio& aOther) const {
+    return !(*this == aOther);
+  }
+
+  bool operator<(const AspectRatio& aOther) const {
+    MOZ_ASSERT(
+        mUseBoxSizing == aOther.mUseBoxSizing,
+        "Do not compare AspectRatio if their mUseBoxSizing are different.");
+    return mRatio < aOther.mRatio;
+  }
+
+ private:
+  // 0.0f represents no aspect ratio.
+  float mRatio = 0.0f;
+  UseBoxSizing mUseBoxSizing = UseBoxSizing::No;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_AspectRatio_h