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diff --git a/layout/base/Baseline.h b/layout/base/Baseline.h
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+/* 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 LAYOUT_BASE_BASELINE_H_
+#define LAYOUT_BASE_BASELINE_H_
+
+#include "nsCoord.h"
+#include "mozilla/WritingModes.h"
+
+class nsIFrame;
+
+namespace mozilla {
+
+// https://drafts.csswg.org/css-align-3/#baseline-sharing-group
+enum class BaselineSharingGroup : uint8_t {
+  // NOTE Used as an array index so must be 0 and 1.
+  First = 0,
+  Last = 1,
+};
+
+// Layout context under which the baseline is being exported to.
+enum class BaselineExportContext : uint8_t {
+  LineLayout = 0,
+  Other = 1,
+};
+
+class Baseline {
+ public:
+  /**
+   * Synthesize a first(last) inline-axis baseline in aWM based on aFrame's
+   * margin-box.
+   *
+   * An alphabetical baseline is at the end edge of aFrame's margin-box with
+   * respect to aWM's block-axis, and a central baseline is halfway between the
+   * start and end edges. (aWM tells which baseline to use.)
+   * https://drafts.csswg.org/css-align-3/#synthesize-baseline
+   *
+   * @note This works only when aFrame's writing-mode is parallel to aWM.
+   * @param aWM the writing-mode of the alignment context.
+   * @return an offset from aFrame's border-box start(end) edge in aWM's
+   *         block-axis for a first(last) baseline, respectively.
+   */
+  static nscoord SynthesizeBOffsetFromMarginBox(const nsIFrame* aFrame,
+                                                WritingMode aWM,
+                                                BaselineSharingGroup);
+
+  /**
+   * Synthesize a first(last) inline-axis baseline in aWM based on aFrame's
+   * border-box.
+   *
+   * An alphabetical baseline is at the end edge of aFrame's border-box with
+   * respect to aWM's block-axis, and a central baseline is halfway between the
+   * start and end edges. (aWM tells which baseline to use.)
+   * https://drafts.csswg.org/css-align-3/#synthesize-baseline
+   *
+   * @param aWM the writing-mode of the alignment context.
+   * @return an offset from aFrame's border-box start(end) edge in aWM's
+   *         block-axis for a first(last) baseline, respectively.
+   */
+  static nscoord SynthesizeBOffsetFromBorderBox(const nsIFrame* aFrame,
+                                                WritingMode aWM,
+                                                BaselineSharingGroup);
+  /**
+   * As above, but using the content box.
+   */
+  static nscoord SynthesizeBOffsetFromContentBox(const nsIFrame*, WritingMode,
+                                                 BaselineSharingGroup);
+  /**
+   * As above, but using the padding box.
+   */
+  static nscoord SynthesizeBOffsetFromPaddingBox(const nsIFrame*, WritingMode,
+                                                 BaselineSharingGroup);
+};
+
+}  // namespace mozilla
+
+#endif  // LAYOUT_BASE_BASELINE_H_