diff --git a/layout/generic/nsIFrame.h b/layout/generic/nsIFrame.h
index 4c2e4fd84b7f..3217f8b5c977 100644
--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -787,12 +787,13 @@ public:
    * accepts an *optional* pointer the style struct.
    */
   #define STYLE_STRUCT(name_, checkdata_cb_)                                  \
-    const nsStyle##name_ * Style##name_ () const {                            \
+    const nsStyle##name_ * Style##name_ () const MOZ_NONNULL_RETURN {         \
       NS_ASSERTION(mStyleContext, "No style context found!");                 \
       return mStyleContext->Style##name_ ();                                  \
     }                                                                         \
     const nsStyle##name_ * Style##name_##WithOptionalParam(                   \
-                             const nsStyle##name_ * aStyleStruct) const {     \
+                             const nsStyle##name_ * aStyleStruct) const       \
+                             MOZ_NONNULL_RETURN {                             \
       if (aStyleStruct) {                                                     \
         MOZ_ASSERT(aStyleStruct == Style##name_());                           \
         return aStyleStruct;                                                  \
diff --git a/layout/style/nsStyleContext.h b/layout/style/nsStyleContext.h
index 632f1ac3dc57..161e8ea31906 100644
--- a/layout/style/nsStyleContext.h
+++ b/layout/style/nsStyleContext.h
@@ -345,7 +345,7 @@ public:
    * function, both because they're easier to read and because they're
    * faster.
    */
-  const void* NS_FASTCALL StyleData(nsStyleStructID aSID);
+  const void* NS_FASTCALL StyleData(nsStyleStructID aSID) MOZ_NONNULL_RETURN;
 
   /**
    * Define typesafe getter functions for each style struct by
@@ -354,9 +354,9 @@ public:
    *   const nsStyleBorder* StyleBorder();
    *   const nsStyleColor* StyleColor();
    */
-  #define STYLE_STRUCT(name_, checkdata_cb_)              \
-    const nsStyle##name_ * Style##name_() {               \
-      return DoGetStyle##name_<true>();                   \
+  #define STYLE_STRUCT(name_, checkdata_cb_)                   \
+    const nsStyle##name_ * Style##name_() MOZ_NONNULL_RETURN { \
+      return DoGetStyle##name_<true>();                        \
     }
   #include "nsStyleStructList.h"
   #undef STYLE_STRUCT