--- a/dom/base/nsGkAtomList.h
+++ b/dom/base/nsGkAtomList.h
@@ -1324,16 +1324,17 @@ GK_ATOM(visuallyselected, "visuallyselec
 GK_ATOM(vlink, "vlink")
 GK_ATOM(vspace, "vspace")
 GK_ATOM(wbr, "wbr")
 GK_ATOM(webkitdirectory, "webkitdirectory")
 GK_ATOM(when, "when")
 GK_ATOM(where, "where")
 GK_ATOM(widget, "widget")
 GK_ATOM(width, "width")
+GK_ATOM(willChange, "will-change")
 GK_ATOM(window, "window")
 GK_ATOM(headerWindowTarget, "window-target")
 GK_ATOM(windowtype, "windowtype")
 GK_ATOM(withParam, "with-param")
 GK_ATOM(wizard, "wizard")
 GK_ATOM(wrap, "wrap")
 GK_ATOM(headerDNSPrefetchControl,"x-dns-prefetch-control")
 GK_ATOM(headerCSP, "content-security-policy")

--- a/layout/base/nsLayoutDebugger.cpp
+++ b/layout/base/nsLayoutDebugger.cpp
@@ -164,23 +164,25 @@ PrintDisplayItemTo(nsDisplayListBuilder*
           aItem->IsUniform(aBuilder) ? " uniform" : "",
           aItem->ReferenceFrame(), aItem->GetAnimatedGeometryRoot()->mFrame);
 
   for (auto iter = opaque.RectIter(); !iter.Done(); iter.Next()) {
     const nsRect& r = iter.Get();
     aStream << nsPrintfCString(" (opaque %d,%d,%d,%d)", r.x, r.y, r.width, r.height);
   }
 
-  if (aItem->Frame()->StyleDisplay()->mWillChange.Length() > 0) {
+  const auto& willChange = aItem->Frame()->StyleDisplay()->mWillChange;
+  if (!willChange.IsEmpty()) {
     aStream << " (will-change=";
-    for (size_t i = 0; i < aItem->Frame()->StyleDisplay()->mWillChange.Length(); i++) {
+    for (size_t i = 0; i < willChange.Length(); i++) {
       if (i > 0) {
         aStream << ",";
       }
-      aStream << NS_LossyConvertUTF16toASCII(aItem->Frame()->StyleDisplay()->mWillChange[i]).get();
+      nsDependentAtomString buffer(willChange[i]);
+      aStream << NS_LossyConvertUTF16toASCII(buffer).get();
     }
     aStream << ")";
   }
 
   // Display item specific debug info
   aItem->WriteDebugInfo(aStream);
 
 #ifdef MOZ_DUMP_PAINTING

--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -16093,24 +16093,23 @@ bool CSSParserImpl::ParseWillChange()
       if (first) {
         AppendValue(eCSSProperty_will_change, value);
         return true;
       } else {
         return false;
       }
     }
 
-    nsString str;
-    value.GetStringValue(str);
-    if (str.LowerCaseEqualsLiteral("default") ||
-        str.LowerCaseEqualsLiteral("will-change")) {
-      return false;
-    }
-
-    currentListValue->mValue = value;
+    value.AtomizeIdentValue();
+    nsIAtom* atom = value.GetAtomValue();
+    if (atom == nsGkAtoms::_default || atom == nsGkAtoms::willChange) {
+      return false;
+    }
+
+    currentListValue->mValue = Move(value);
 
     if (!ExpectSymbol(',', true)) {
       break;
     }
     currentListValue->mNext = new nsCSSValueList;
     currentListValue = currentListValue->mNext;
     first = false;
   }

--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -4830,29 +4830,28 @@ nsComputedDOMStyle::DoGetClip()
   }
 
   return val.forget();
 }
 
 already_AddRefed<CSSValue>
 nsComputedDOMStyle::DoGetWillChange()
 {
-  const nsTArray<nsString>& willChange = StyleDisplay()->mWillChange;
+  const nsCOMArray<nsIAtom>& willChange = StyleDisplay()->mWillChange;
 
   if (willChange.IsEmpty()) {
     RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
     val->SetIdent(eCSSKeyword_auto);
     return val.forget();
   }
 
   RefPtr<nsDOMCSSValueList> valueList = GetROCSSValueList(true);
-  for (size_t i = 0; i < willChange.Length(); i++) {
-    const nsString& willChangeIdentifier = willChange[i];
+  for (const nsIAtom* ident : willChange) {
     RefPtr<nsROCSSPrimitiveValue> property = new nsROCSSPrimitiveValue;
-    property->SetString(willChangeIdentifier);
+    property->SetString(nsDependentAtomString(ident));
     valueList->AppendCSSValue(property.forget());
   }
 
   return valueList.forget();
 }
 
 already_AddRefed<CSSValue>
 nsComputedDOMStyle::DoGetOverflow()

--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -6459,54 +6459,50 @@ nsRuleNode::ComputeDisplayData(void* aSt
 
   case eCSSUnit_List:
   case eCSSUnit_ListDep: {
     display->mWillChange.Clear();
     display->mWillChangeBitField = 0;
     for (const nsCSSValueList* item = willChangeValue->GetListValue();
          item; item = item->mNext)
     {
-      if (item->mValue.UnitHasStringValue()) {
-        nsAutoString buffer;
-        item->mValue.GetStringValue(buffer);
-        display->mWillChange.AppendElement(buffer);
-
-        if (buffer.EqualsLiteral("transform")) {
-          display->mWillChangeBitField |= NS_STYLE_WILL_CHANGE_TRANSFORM;
-        }
-        if (buffer.EqualsLiteral("opacity")) {
-          display->mWillChangeBitField |= NS_STYLE_WILL_CHANGE_OPACITY;
-        }
-        if (buffer.EqualsLiteral("scroll-position")) {
-          display->mWillChangeBitField |= NS_STYLE_WILL_CHANGE_SCROLL;
-        }
-
-        nsCSSPropertyID prop =
-          nsCSSProps::LookupProperty(buffer, CSSEnabledState::eForAllContent);
-        if (prop != eCSSProperty_UNKNOWN &&
-            prop != eCSSPropertyExtra_variable) {
-          // If the property given is a shorthand, it indicates the expectation
-          // for all the longhands the shorthand expands to.
-          if (nsCSSProps::IsShorthand(prop)) {
-            for (const nsCSSPropertyID* shorthands =
-                   nsCSSProps::SubpropertyEntryFor(prop);
-                 *shorthands != eCSSProperty_UNKNOWN; ++shorthands) {
-              display->mWillChangeBitField |= GetWillChangeBitFieldFromPropFlags(*shorthands);
-            }
-          } else {
-            display->mWillChangeBitField |= GetWillChangeBitFieldFromPropFlags(prop);
+      nsIAtom* atom = item->mValue.GetAtomValue();
+      display->mWillChange.AppendElement(atom);
+      if (atom == nsGkAtoms::transform) {
+        display->mWillChangeBitField |= NS_STYLE_WILL_CHANGE_TRANSFORM;
+      } else if (atom == nsGkAtoms::opacity) {
+        display->mWillChangeBitField |= NS_STYLE_WILL_CHANGE_OPACITY;
+      } else if (atom == nsGkAtoms::scrollPosition) {
+        display->mWillChangeBitField |= NS_STYLE_WILL_CHANGE_SCROLL;
+      }
+
+      nsDependentAtomString buffer(atom);
+      nsCSSPropertyID prop =
+        nsCSSProps::LookupProperty(buffer, CSSEnabledState::eForAllContent);
+      if (prop != eCSSProperty_UNKNOWN &&
+          prop != eCSSPropertyExtra_variable) {
+        // If the property given is a shorthand, it indicates the expectation
+        // for all the longhands the shorthand expands to.
+        if (nsCSSProps::IsShorthand(prop)) {
+          for (const nsCSSPropertyID* shorthands =
+                  nsCSSProps::SubpropertyEntryFor(prop);
+                *shorthands != eCSSProperty_UNKNOWN; ++shorthands) {
+            display->mWillChangeBitField |= GetWillChangeBitFieldFromPropFlags(*shorthands);
           }
+        } else {
+          display->mWillChangeBitField |= GetWillChangeBitFieldFromPropFlags(prop);
         }
       }
     }
     break;
   }
 
   case eCSSUnit_Inherit:
-    display->mWillChange = parentDisplay->mWillChange;
+    display->mWillChange.Clear();
+    display->mWillChange.AppendElements(parentDisplay->mWillChange);
     display->mWillChangeBitField = parentDisplay->mWillChangeBitField;
     conditions.SetUncacheable();
     break;
 
   case eCSSUnit_Initial:
   case eCSSUnit_Unset:
   case eCSSUnit_Auto:
     display->mWillChange.Clear();

--- a/layout/style/nsStyleStruct.h
+++ b/layout/style/nsStyleStruct.h
@@ -2860,17 +2860,17 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsSt
   uint8_t mIsolation;           // [reset] see nsStyleConsts.h
   uint8_t mTopLayer;            // [reset] see nsStyleConsts.h
   uint8_t mWillChangeBitField;  // [reset] see nsStyleConsts.h. Stores a
                                 // bitfield representation of the properties
                                 // that are frequently queried. This should
                                 // match mWillChange. Also tracks if any of the
                                 // properties in the will-change list require
                                 // a stacking context.
-  nsTArray<nsString> mWillChange;
+  nsCOMArray<nsIAtom> mWillChange;
 
   uint8_t mTouchAction;         // [reset] see nsStyleConsts.h
   uint8_t mScrollBehavior;      // [reset] see nsStyleConsts.h NS_STYLE_SCROLL_BEHAVIOR_*
   uint8_t mScrollSnapTypeX;     // [reset] see nsStyleConsts.h NS_STYLE_SCROLL_SNAP_TYPE_*
   uint8_t mScrollSnapTypeY;     // [reset] see nsStyleConsts.h NS_STYLE_SCROLL_SNAP_TYPE_*
   nsStyleCoord mScrollSnapPointsX; // [reset]
   nsStyleCoord mScrollSnapPointsY; // [reset]
   mozilla::Position mScrollSnapDestination; // [reset]