--- a/layout/base/ServoRestyleManager.cpp
+++ b/layout/base/ServoRestyleManager.cpp
@@ -301,18 +301,16 @@ ServoRestyleManager::ReparentStyleContex
 {
   NS_WARNING("stylo: ServoRestyleManager::ReparentStyleContext not implemented");
   return NS_OK;
 }
 
 ServoElementSnapshot*
 ServoRestyleManager::SnapshotForElement(Element* aElement)
 {
-  ServoElementSnapshot* snapshot = mModifiedElements.LookupOrAdd(aElement);
-  if (!snapshot->HasAny(
-        ServoElementSnapshot::Flags::HTMLElementInHTMLDocument)) {
-    snapshot->SetIsHTMLElementInHTMLDocument(
-      aElement->IsHTMLElement() && aElement->OwnerDoc()->IsHTMLDocument());
+  auto* entry = mModifiedElements.PutEntry(aElement);
+  if (!entry->mData) {
+    entry->mData = new ServoElementSnapshot(aElement);
   }
-  return snapshot;
+  return entry->mData;
 }
 
 } // namespace mozilla

--- a/layout/style/ServoElementSnapshot.cpp
+++ b/layout/style/ServoElementSnapshot.cpp
@@ -1,19 +1,30 @@
 /* -*- 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/. */
 
 #include "mozilla/ServoElementSnapshot.h"
 #include "mozilla/dom/Element.h"
+#include "nsIContentInlines.h"
 
 namespace mozilla {
 
+ServoElementSnapshot::ServoElementSnapshot(Element* aElement)
+  : mContains(Flags(0))
+  , mState(0)
+  , mExplicitRestyleHint(nsRestyleHint(0))
+  , mExplicitChangeHint(nsChangeHint(0))
+{
+  mIsHTMLElementInHTMLDocument =
+    aElement->IsHTMLElement() && aElement->IsInHTMLDocument();
+}
+
 void
 ServoElementSnapshot::AddAttrs(Element* aElement)
 {
   MOZ_ASSERT(aElement);
 
   if (!HasAny(Flags::Attributes)) {
     return;
   }

--- a/layout/style/ServoElementSnapshot.h
+++ b/layout/style/ServoElementSnapshot.h
@@ -41,18 +41,17 @@ struct ServoAttrSnapshot
 /**
  * A bitflags enum class used to determine what data does a ServoElementSnapshot
  * contains.
  */
 enum class ServoElementSnapshotFlags : uint8_t
 {
   State = 1 << 0,
   Attributes = 1 << 1,
-  HTMLElementInHTMLDocument = 1 << 2,
-  All = State | Attributes | HTMLElementInHTMLDocument
+  All = State | Attributes
 };
 
 MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(ServoElementSnapshotFlags)
 
 /**
  * This class holds all non-tree-structural state of an element that might be
  * used for selector matching eventually.
  *
@@ -63,27 +62,17 @@ class ServoElementSnapshot
 {
   typedef dom::BorrowedAttrInfo BorrowedAttrInfo;
   typedef dom::Element Element;
   typedef EventStates::ServoType ServoStateType;
 
 public:
   typedef ServoElementSnapshotFlags Flags;
 
-  /**
-   * Empty snapshot, with no data at all.
-   */
-  ServoElementSnapshot()
-    : mContains(Flags(0))
-    , mState(0)
-    , mExplicitRestyleHint(nsRestyleHint(0))
-    , mExplicitChangeHint(nsChangeHint(0))
-    , mIsHTMLElementInHTMLDocument(false)
-  {
-  }
+  explicit ServoElementSnapshot(Element* aElement);
 
   bool HasAttrs() { return HasAny(Flags::Attributes); }
 
   bool HasState() { return HasAny(Flags::State); }
 
   /**
    * Captures the given state (if not previously captured).
    */
@@ -149,24 +138,16 @@ public:
       if (mAttrs[i].mName.Equals(aLocalName, aNamespaceID)) {
         return &mAttrs[i].mValue;
       }
     }
 
     return nullptr;
   }
 
-  void SetIsHTMLElementInHTMLDocument(bool aIs)
-  {
-    MOZ_ASSERT(!HasAny(Flags::HTMLElementInHTMLDocument),
-               "Only expected to be set once!");
-    mContains |= Flags::HTMLElementInHTMLDocument;
-    mIsHTMLElementInHTMLDocument = aIs;
-  }
-
   bool HasAny(Flags aFlags) { return bool(mContains & aFlags); }
 
 private:
   // TODO: Profile, a 1 or 2 element AutoTArray could be worth it, given we know
   // we're dealing with attribute changes when we take snapshots of attributes,
   // though it can be wasted space if we deal with a lot of state-only
   // snapshots.
   Flags mContains;