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diff --git a/dom/svg/SVGPathData.h b/dom/svg/SVGPathData.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 DOM_SVG_SVGPATHDATA_H_
+#define DOM_SVG_SVGPATHDATA_H_
+
+#include "nsCOMPtr.h"
+#include "nsDebug.h"
+#include "nsIContent.h"
+#include "nsINode.h"
+#include "nsIWeakReferenceUtils.h"
+#include "mozilla/dom/SVGElement.h"
+#include "mozilla/gfx/2D.h"
+#include "mozilla/gfx/Types.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RefPtr.h"
+#include "nsTArray.h"
+
+#include <string.h>
+
+namespace mozilla {
+
+struct StylePathCommand;
+struct SVGMark;
+enum class StyleStrokeLinecap : uint8_t;
+
+class SVGPathDataParser;  // IWYU pragma: keep
+
+namespace dom {
+class DOMSVGPathSeg;
+class DOMSVGPathSegList;
+}  // namespace dom
+
+/**
+ * ATTENTION! WARNING! WATCH OUT!!
+ *
+ * Consumers that modify objects of this type absolutely MUST keep the DOM
+ * wrappers for those lists (if any) in sync!! That's why this class is so
+ * locked down.
+ *
+ * The DOM wrapper class for this class is DOMSVGPathSegList.
+ *
+ * This class is not called |class SVGPathSegList| for one very good reason;
+ * this class does not provide a list of "SVGPathSeg" items, it provides an
+ * array of floats into which path segments are encoded. See the paragraphs
+ * that follow for why. Note that the Length() method returns the number of
+ * floats in our array, not the number of encoded segments, and the index
+ * operator indexes floats in the array, not segments. If this class were
+ * called SVGPathSegList the names of these methods would be very misleading.
+ *
+ * The reason this class is designed in this way is because there are many
+ * different types of path segment, each taking a different numbers of
+ * arguments. We want to store the segments in an nsTArray to avoid individual
+ * allocations for each item, but the different size of segments means we can't
+ * have one single segment type for the nsTArray (not without using a space
+ * wasteful union or something similar). Since the internal code does not need
+ * to index into the list (the DOM wrapper does, but it handles that itself)
+ * the obvious solution is to have the items in this class take up variable
+ * width and have the internal code iterate over these lists rather than index
+ * into them.
+ *
+ * Implementing indexing to segments with O(1) performance would require us to
+ * allocate and maintain a separate segment index table (keeping that table in
+ * sync when items are inserted or removed from the list). So long as the
+ * internal code doesn't require indexing to segments, we can avoid that
+ * overhead and additional complexity.
+ *
+ * Segment encoding: the first float in the encoding of a segment contains the
+ * segment's type. The segment's type is encoded to/decoded from this float
+ * using the static methods SVGPathSegUtils::EncodeType(uint32_t)/
+ * SVGPathSegUtils::DecodeType(float). If the path segment type in question
+ * takes any arguments then these follow the first float, and are in the same
+ * order as they are given in a <path> element's 'd' attribute (NOT in the
+ * order of the createSVGPathSegXxx() methods' arguments from the SVG DOM
+ * interface SVGPathElement, which are different...grr). Consumers can use
+ * SVGPathSegUtils::ArgCountForType(type) to determine how many arguments
+ * there are (if any), and thus where the current encoded segment ends, and
+ * where the next segment (if any) begins.
+ */
+class SVGPathData {
+  friend class SVGAnimatedPathSegList;
+  friend class dom::DOMSVGPathSeg;
+  friend class dom::DOMSVGPathSegList;
+  friend class SVGPathDataParser;
+  // SVGPathDataParser will not keep wrappers in sync, so consumers
+  // are responsible for that!
+
+  using DrawTarget = gfx::DrawTarget;
+  using Path = gfx::Path;
+  using PathBuilder = gfx::PathBuilder;
+  using FillRule = gfx::FillRule;
+  using Float = gfx::Float;
+  using CapStyle = gfx::CapStyle;
+
+ public:
+  using const_iterator = const float*;
+
+  SVGPathData() = default;
+  ~SVGPathData() = default;
+
+  // Only methods that don't make/permit modification to this list are public.
+  // Only our friend classes can access methods that may change us.
+
+  /// This may return an incomplete string on OOM, but that's acceptable.
+  void GetValueAsString(nsAString& aValue) const;
+
+  bool IsEmpty() const { return mData.IsEmpty(); }
+
+#ifdef DEBUG
+  /**
+   * This method iterates over the encoded segment data and counts the number
+   * of segments we currently have.
+   */
+  uint32_t CountItems() const;
+#endif
+
+  /**
+   * Returns the number of *floats* in the encoding array, and NOT the number
+   * of segments encoded in this object. (For that, see CountItems() above.)
+   */
+  uint32_t Length() const { return mData.Length(); }
+
+  const float& operator[](uint32_t aIndex) const { return mData[aIndex]; }
+
+  // Used by SMILCompositor to check if the cached base val is out of date
+  bool operator==(const SVGPathData& rhs) const {
+    // We use memcmp so that we don't need to worry that the data encoded in
+    // the first float may have the same bit pattern as a NaN.
+    return mData.Length() == rhs.mData.Length() &&
+           memcmp(mData.Elements(), rhs.mData.Elements(),
+                  mData.Length() * sizeof(float)) == 0;
+  }
+
+  bool SetCapacity(uint32_t aSize) {
+    return mData.SetCapacity(aSize, fallible);
+  }
+
+  void Compact() { mData.Compact(); }
+
+  float GetPathLength() const;
+
+  uint32_t GetPathSegAtLength(float aDistance) const;
+
+  void GetMarkerPositioningData(nsTArray<SVGMark>* aMarks) const;
+
+  /**
+   * Returns true, except on OOM, in which case returns false.
+   */
+  bool GetDistancesFromOriginToEndsOfVisibleSegments(
+      FallibleTArray<double>* aOutput) const;
+
+  /**
+   * This returns a path without the extra little line segments that
+   * ApproximateZeroLengthSubpathSquareCaps can insert if we have square-caps.
+   * See the comment for that function for more info on that.
+   */
+  already_AddRefed<Path> BuildPathForMeasuring() const;
+
+  already_AddRefed<Path> BuildPath(PathBuilder* aBuilder,
+                                   StyleStrokeLinecap aStrokeLineCap,
+                                   Float aStrokeWidth) const;
+  /**
+   * This function tries to build the path from an array of StylePathCommand,
+   * which is generated by cbindgen from Rust (see ServoStyleConsts.h).
+   * Basically, this is a variant of the above BuildPath() functions.
+   */
+  static already_AddRefed<Path> BuildPath(Span<const StylePathCommand> aPath,
+                                          PathBuilder* aBuilder,
+                                          StyleStrokeLinecap aStrokeLineCap,
+                                          Float aStrokeWidth,
+                                          float aZoomFactor = 1.0);
+
+  const_iterator begin() const { return mData.Elements(); }
+  const_iterator end() const { return mData.Elements() + mData.Length(); }
+
+  // memory reporting methods
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  // Access to methods that can modify objects of this type is deliberately
+  // limited. This is to reduce the chances of someone modifying objects of
+  // this type without taking the necessary steps to keep DOM wrappers in sync.
+  // If you need wider access to these methods, consider adding a method to
+  // SVGAnimatedPathSegList and having that class act as an intermediary so it
+  // can take care of keeping DOM wrappers in sync.
+
+ protected:
+  using iterator = float*;
+
+  /**
+   * This may fail on OOM if the internal capacity needs to be increased, in
+   * which case the list will be left unmodified.
+   */
+  nsresult CopyFrom(const SVGPathData& rhs);
+
+  float& operator[](uint32_t aIndex) { return mData[aIndex]; }
+
+  /**
+   * This may fail (return false) on OOM if the internal capacity is being
+   * increased, in which case the list will be left unmodified.
+   */
+  bool SetLength(uint32_t aLength) {
+    return mData.SetLength(aLength, fallible);
+  }
+
+  nsresult SetValueFromString(const nsAString& aValue);
+
+  void Clear() { mData.Clear(); }
+
+  // Our DOM wrappers have direct access to our mData, so they directly
+  // manipulate it rather than us implementing:
+  //
+  // * InsertItem(uint32_t aDataIndex, uint32_t aType, const float *aArgs);
+  // * ReplaceItem(uint32_t aDataIndex, uint32_t aType, const float *aArgs);
+  // * RemoveItem(uint32_t aDataIndex);
+  // * bool AppendItem(uint32_t aType, const float *aArgs);
+
+  nsresult AppendSeg(uint32_t aType, ...);  // variable number of float args
+
+  iterator begin() { return mData.Elements(); }
+  iterator end() { return mData.Elements() + mData.Length(); }
+
+  FallibleTArray<float> mData;
+};
+
+/**
+ * This SVGPathData subclass is for SVGPathSegListSMILType which needs to
+ * have write access to the lists it works with.
+ *
+ * Instances of this class do not have DOM wrappers that need to be kept in
+ * sync, so we can safely expose any protected base class methods required by
+ * the SMIL code.
+ */
+class SVGPathDataAndInfo final : public SVGPathData {
+ public:
+  explicit SVGPathDataAndInfo(dom::SVGElement* aElement = nullptr)
+      : mElement(do_GetWeakReference(static_cast<nsINode*>(aElement))) {}
+
+  void SetElement(dom::SVGElement* aElement) {
+    mElement = do_GetWeakReference(static_cast<nsINode*>(aElement));
+  }
+
+  dom::SVGElement* Element() const {
+    nsCOMPtr<nsIContent> e = do_QueryReferent(mElement);
+    return static_cast<dom::SVGElement*>(e.get());
+  }
+
+  nsresult CopyFrom(const SVGPathDataAndInfo& rhs) {
+    mElement = rhs.mElement;
+    return SVGPathData::CopyFrom(rhs);
+  }
+
+  /**
+   * Returns true if this object is an "identity" value, from the perspective
+   * of SMIL. In other words, returns true until the initial value set up in
+   * SVGPathSegListSMILType::Init() has been changed with a SetElement() call.
+   */
+  bool IsIdentity() const {
+    if (!mElement) {
+      MOZ_ASSERT(IsEmpty(), "target element propagation failure");
+      return true;
+    }
+    return false;
+  }
+
+  /**
+   * Exposed so that SVGPathData baseVals can be copied to
+   * SVGPathDataAndInfo objects. Note that callers should also call
+   * SetElement() when using this method!
+   */
+  using SVGPathData::CopyFrom;
+
+  // Exposed since SVGPathData objects can be modified.
+  using SVGPathData::iterator;
+  using SVGPathData::operator[];
+  using SVGPathData::begin;
+  using SVGPathData::end;
+  using SVGPathData::SetLength;
+
+ private:
+  // We must keep a weak reference to our element because we may belong to a
+  // cached baseVal SMILValue. See the comments starting at:
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=515116#c15
+  // See also https://bugzilla.mozilla.org/show_bug.cgi?id=653497
+  nsWeakPtr mElement;
+};
+
+}  // namespace mozilla
+
+#endif  // DOM_SVG_SVGPATHDATA_H_