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diff --git a/layout/generic/CSSAlignUtils.h b/layout/generic/CSSAlignUtils.h
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+++ b/layout/generic/CSSAlignUtils.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/. */
+
+/* Utility code for performing CSS Box Alignment */
+
+#ifndef mozilla_CSSAlignUtils_h
+#define mozilla_CSSAlignUtils_h
+
+#include "mozilla/WritingModes.h"
+
+namespace mozilla {
+
+struct ReflowInput;
+struct StyleAlignFlags;
+
+class CSSAlignUtils {
+ public:
+  /**
+   * Flags to customize the behavior of AlignJustifySelf:
+   */
+  enum class AlignJustifyFlags {
+    NoFlags = 0,
+    // Indicates that we have <overflow-position> = safe.
+    OverflowSafe = 1 << 0,
+    // Indicates that the container's start side in aAxis is the same
+    // as the child's start side in the child's parallel axis.
+    SameSide = 1 << 1,
+    // Indicates that AlignJustifySelf() shouldn't expand "auto" margins.
+    // (By default, AlignJustifySelf() *will* expand such margins, to fill the
+    // available space before any alignment is done.)
+    IgnoreAutoMargins = 1 << 2,
+  };
+
+  /**
+   * This computes the aligned offset of a CSS-aligned child within its
+   * alignment container. The returned offset is distance between the
+   * logical "start" edge of the alignment container & the logical "start" edge
+   * of the aligned child (in terms of the alignment container's writing mode).
+   *
+   * @param aAlignment An enumerated value representing a keyword for
+   *                   "align-self" or "justify-self". The values
+   *                   StyleAlignFlags::{AUTO,LEFT,RIGHT} must *not* be
+   *                   passed here; this method expects the caller to have
+   *                   already resolved those to 'start', 'end', or 'stretch'.
+   * @param aAxis The container's axis in which we're doing alignment.
+   * @param aBaselineAdjust The amount to offset baseline-aligned children.
+   * @param aCBSize The size of the alignment container, in its aAxis.
+   * @param aRI A ReflowInput for the child.
+   * @param aChildSize The child's LogicalSize (in its own writing mode).
+   */
+  static nscoord AlignJustifySelf(const StyleAlignFlags& aAlignment,
+                                  LogicalAxis aAxis, AlignJustifyFlags aFlags,
+                                  nscoord aBaselineAdjust, nscoord aCBSize,
+                                  const ReflowInput& aRI,
+                                  const LogicalSize& aChildSize);
+};
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(CSSAlignUtils::AlignJustifyFlags)
+
+}  // namespace mozilla
+
+#endif  // mozilla_CSSAlignUtils_h