--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -11431,16 +11431,22 @@ class CGDOMJSProxyHandler_defineProperty
 
         indexedSetter = self.descriptor.operations['IndexedSetter']
         if indexedSetter:
             set += fill(
                 """
                 uint32_t index = GetArrayIndexFromId(cx, id);
                 if (IsArrayIndex(index)) {
                   *defined = true;
+                  // https://heycam.github.io/webidl/#legacy-platform-object-defineownproperty
+                  // Step 1.1.  The no-indexed-setter case is handled by step 1.2.
+                  if (!desc.isDataDescriptor()) {
+                    return opresult.failNotDataDescriptor();
+                  }
+
                   $*{callSetter}
                   return opresult.succeed();
                 }
                 """,
                 callSetter=CGProxyIndexedSetter(self.descriptor).define())
         elif self.descriptor.supportsIndexedProperties():
             # We allow untrusted content to prevent Xrays from setting a
             # property if that property is an indexed property and we have no

--- a/js/public/Class.h
+++ b/js/public/Class.h
@@ -197,16 +197,17 @@ class ObjectOpResult
     JS_PUBLIC_API(bool) failCantSetInterposed();
     JS_PUBLIC_API(bool) failCantDefineWindowElement();
     JS_PUBLIC_API(bool) failCantDeleteWindowElement();
     JS_PUBLIC_API(bool) failCantDeleteWindowNamedProperty();
     JS_PUBLIC_API(bool) failCantPreventExtensions();
     JS_PUBLIC_API(bool) failCantSetProto();
     JS_PUBLIC_API(bool) failNoNamedSetter();
     JS_PUBLIC_API(bool) failNoIndexedSetter();
+    JS_PUBLIC_API(bool) failNotDataDescriptor();
 
     uint32_t failureCode() const {
         MOZ_ASSERT(!ok());
         return uint32_t(code_);
     }
 
     /*
      * Report an error or warning if necessary; return true to proceed and

--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -583,16 +583,17 @@ MSG_DEF(JSMSG_ATOMICS_WAIT_NOT_ALLOWED, 
 // XPConnect wrappers and DOM bindings
 MSG_DEF(JSMSG_CANT_SET_INTERPOSED,       1, JSEXN_TYPEERR, "unable to set interposed data property '{0}'")
 MSG_DEF(JSMSG_CANT_DEFINE_WINDOW_ELEMENT, 0, JSEXN_TYPEERR, "can't define elements on a Window object")
 MSG_DEF(JSMSG_CANT_DELETE_WINDOW_ELEMENT, 0, JSEXN_TYPEERR, "can't delete elements from a Window object")
 MSG_DEF(JSMSG_CANT_DELETE_WINDOW_NAMED_PROPERTY, 1, JSEXN_TYPEERR, "can't delete property {0} from window's named properties object")
 MSG_DEF(JSMSG_CANT_PREVENT_EXTENSIONS,   0, JSEXN_TYPEERR, "can't prevent extensions on this proxy object")
 MSG_DEF(JSMSG_NO_NAMED_SETTER,           2, JSEXN_TYPEERR, "{0} doesn't have a named property setter for '{1}'")
 MSG_DEF(JSMSG_NO_INDEXED_SETTER,         2, JSEXN_TYPEERR, "{0} doesn't have an indexed property setter for '{1}'")
+MSG_DEF(JSMSG_NOT_DATA_DESCRIPTOR,       2, JSEXN_TYPEERR, "can't define a getter/setter for element '{1}' of {0} object")
 
 // Super
 MSG_DEF(JSMSG_CANT_DELETE_SUPER, 0, JSEXN_REFERENCEERR, "invalid delete involving 'super'")
 MSG_DEF(JSMSG_REINIT_THIS,       0, JSEXN_REFERENCEERR, "super() called twice in derived class constructor")
 
 // Modules
 MSG_DEF(JSMSG_BAD_DEFAULT_EXPORT,        0, JSEXN_SYNTAXERR, "default export cannot be provided by export *")
 MSG_DEF(JSMSG_MISSING_INDIRECT_EXPORT,   0, JSEXN_SYNTAXERR, "indirect export not found")

--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -274,16 +274,22 @@ JS::ObjectOpResult::failNoNamedSetter()
 }
 
 JS_PUBLIC_API(bool)
 JS::ObjectOpResult::failNoIndexedSetter()
 {
     return fail(JSMSG_NO_INDEXED_SETTER);
 }
 
+JS_PUBLIC_API(bool)
+JS::ObjectOpResult::failNotDataDescriptor()
+{
+    return fail(JSMSG_NOT_DATA_DESCRIPTOR);
+}
+
 JS_PUBLIC_API(int64_t)
 JS_Now()
 {
     return PRMJ_Now();
 }
 
 JS_PUBLIC_API(Value)
 JS_GetNaNValue(JSContext* cx)

deleted file mode 100644
--- a/testing/web-platform/meta/WebIDL/ecmascript-binding/legacy-platform-object.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[legacy-platform-object.html]
-  [Test [[DefineOwnProperty\]\] with indexed property setter support.]
-    expected: FAIL
-