diff --git a/.circleci/config.yml b/.circleci/config.yml
index 2d002b2ff5..09a50e9461 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -61,7 +61,7 @@ commands:
       enable_native_custom_element_lifecycle:
         type: boolean
         default: false
-      disable_aria_reflection_polyfill:
+      enable_aria_reflection_global_polyfill:
         type: boolean
         default: false
       node_env_production:
@@ -95,7 +95,7 @@ commands:
             <<# parameters.disable_synthetic >> DISABLE_SYNTHETIC=1 <</ parameters.disable_synthetic >> \
             <<# parameters.force_native_shadow_mode >> FORCE_NATIVE_SHADOW_MODE_FOR_TEST=1 <</ parameters.force_native_shadow_mode >> \
             <<# parameters.enable_native_custom_element_lifecycle >> ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE=1 <</ parameters.enable_native_custom_element_lifecycle >> \
-            <<# parameters.disable_aria_reflection_polyfill >> DISABLE_ARIA_REFLECTION_POLYFILL=1 <</ parameters.disable_aria_reflection_polyfill >> \
+            <<# parameters.enable_aria_reflection_global_polyfill >> ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL=1 <</ parameters.enable_aria_reflection_global_polyfill >> \
             <<# parameters.node_env_production >> NODE_ENV_FOR_TEST=production <</ parameters.node_env_production >> \
             <<# parameters.disable_synthetic_shadow_support_in_compiler >> DISABLE_SYNTHETIC_SHADOW_SUPPORT_IN_COMPILER=1 <</ parameters.disable_synthetic_shadow_support_in_compiler >> \
             <<# parameters.legacy_browsers >> LEGACY_BROWSERS=1 <</ parameters.legacy_browsers >> \
@@ -188,7 +188,7 @@ commands:
       enable_native_custom_element_lifecycle:
         type: boolean
         default: false
-      disable_aria_reflection_polyfill:
+      enable_aria_reflection_global_polyfill:
         type: boolean
         default: false
       node_env_production:
@@ -210,7 +210,7 @@ commands:
           disable_synthetic: << parameters.disable_synthetic >>
           force_native_shadow_mode: << parameters.force_native_shadow_mode >>
           enable_native_custom_element_lifecycle: << parameters.enable_native_custom_element_lifecycle >>
-          disable_aria_reflection_polyfill: << parameters.disable_aria_reflection_polyfill >>
+          enable_aria_reflection_global_polyfill: << parameters.enable_aria_reflection_global_polyfill >>
           node_env_production: << parameters.node_env_production >>
           disable_synthetic_shadow_support_in_compiler: << parameters.disable_synthetic_shadow_support_in_compiler >>
           api_version: << parameters.api_version >>
@@ -303,9 +303,11 @@ jobs:
       - run_karma:
           disable_synthetic: true
           api_version: 59
+      - run_karma:
+          enable_aria_reflection_global_polyfill: true
       - run_karma:
           disable_synthetic: true
-          disable_aria_reflection_polyfill: true
+          enable_aria_reflection_global_polyfill: true
       - run_karma:
           disable_synthetic: true
           disable_synthetic_shadow_support_in_compiler: true
diff --git a/packages/@lwc/aria-reflection/README.md b/packages/@lwc/aria-reflection/README.md
index c6f9b1d798..36acebb564 100644
--- a/packages/@lwc/aria-reflection/README.md
+++ b/packages/@lwc/aria-reflection/README.md
@@ -1,5 +1,8 @@
 # @lwc/aria-reflection
 
+> **Note:** use this code at your own risk. It is optimized for backwards-compatibility, not
+> as a forward-looking polyfill that keeps up to date with web standards.
+
 Polyfill for [ARIA string reflection](https://wicg.github.io/aom/spec/aria-reflection.html) on Elements.
 This is part of the [Accessibility Object Model](https://wicg.github.io/aom/explainer.html) (AOM).
 
@@ -21,20 +24,10 @@ npm install @lwc/aria-reflection
 ```
 
 ```js
-import { applyAriaReflection } from '@lwc/aria-reflection';
-
-applyAriaReflection();
-```
-
-The polyfill is applied as soon as the function is executed.
-
-Optionally, you can pass in a custom prototype:
-
-```js
-applyAriaReflection(MyCustomElement.prototype);
+import '@lwc/aria-reflection';
 ```
 
-By default, the polyfill is applied to the global `Element.prototype`.
+The polyfill is applied globally to `Element.prototype` as soon as the module is imported.
 
 ## Implementation
 
diff --git a/packages/@lwc/aria-reflection/package.json b/packages/@lwc/aria-reflection/package.json
index 65ef164c88..8ae1d4ab92 100644
--- a/packages/@lwc/aria-reflection/package.json
+++ b/packages/@lwc/aria-reflection/package.json
@@ -45,7 +45,5 @@
             }
         }
     },
-    "dependencies": {
-        "@lwc/shared": "3.8.0"
-    }
+    "dependencies": {}
 }
diff --git a/packages/@lwc/aria-reflection/src/detect.ts b/packages/@lwc/aria-reflection/src/detect.ts
deleted file mode 100644
index 0baad671c5..0000000000
--- a/packages/@lwc/aria-reflection/src/detect.ts
+++ /dev/null
@@ -1,11 +0,0 @@
-/*
- * Copyright (c) 2018, salesforce.com, inc.
- * All rights reserved.
- * SPDX-License-Identifier: MIT
- * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
- */
-import { getOwnPropertyDescriptor, isUndefined } from '@lwc/shared';
-
-export function detect(propName: string, prototype: any): boolean {
-    return isUndefined(getOwnPropertyDescriptor(prototype, propName));
-}
diff --git a/packages/@lwc/aria-reflection/src/index.ts b/packages/@lwc/aria-reflection/src/index.ts
index b58431d7d6..efad8f79d3 100644
--- a/packages/@lwc/aria-reflection/src/index.ts
+++ b/packages/@lwc/aria-reflection/src/index.ts
@@ -1,20 +1,85 @@
 /*
- * Copyright (c) 2018, salesforce.com, inc.
+ * Copyright (c) 2023, salesforce.com, inc.
  * All rights reserved.
  * SPDX-License-Identifier: MIT
  * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
  */
-import { AriaPropNameToAttrNameMap, keys } from '@lwc/shared';
-import { detect } from './detect';
-import { patch } from './polyfill';
 
-export function applyAriaReflection(prototype: any = Element.prototype) {
-    const ElementPrototypeAriaPropertyNames = keys(AriaPropNameToAttrNameMap);
+// Minimal polyfill of ARIA string reflection, plus some non-standard ARIA props
+// Taken from https://github.com/salesforce/lwc/blob/44a01ef/packages/%40lwc/shared/src/aria.ts#L22-L70
+// This is designed for maximum backwards compatibility on LEX - it should never change.
+// We deliberately don't import from @lwc/shared because that would make this code less portable.
+const ARIA_PROPERTIES = [
+    'ariaActiveDescendant',
+    'ariaAtomic',
+    'ariaAutoComplete',
+    'ariaBusy',
+    'ariaChecked',
+    'ariaColCount',
+    'ariaColIndex',
+    'ariaColSpan',
+    'ariaControls',
+    'ariaCurrent',
+    'ariaDescribedBy',
+    'ariaDetails',
+    'ariaDisabled',
+    'ariaErrorMessage',
+    'ariaExpanded',
+    'ariaFlowTo',
+    'ariaHasPopup',
+    'ariaHidden',
+    'ariaInvalid',
+    'ariaKeyShortcuts',
+    'ariaLabel',
+    'ariaLabelledBy',
+    'ariaLevel',
+    'ariaLive',
+    'ariaModal',
+    'ariaMultiLine',
+    'ariaMultiSelectable',
+    'ariaOrientation',
+    'ariaOwns',
+    'ariaPlaceholder',
+    'ariaPosInSet',
+    'ariaPressed',
+    'ariaReadOnly',
+    'ariaRelevant',
+    'ariaRequired',
+    'ariaRoleDescription',
+    'ariaRowCount',
+    'ariaRowIndex',
+    'ariaRowSpan',
+    'ariaSelected',
+    'ariaSetSize',
+    'ariaSort',
+    'ariaValueMax',
+    'ariaValueMin',
+    'ariaValueNow',
+    'ariaValueText',
+    'role',
+];
 
-    for (let i = 0, len = ElementPrototypeAriaPropertyNames.length; i < len; i += 1) {
-        const propName = ElementPrototypeAriaPropertyNames[i];
-        if (detect(propName, prototype)) {
-            patch(propName, prototype);
-        }
+for (const prop of ARIA_PROPERTIES) {
+    const attribute = prop.replace(/^aria/, 'aria-').toLowerCase(); // e.g. ariaPosInSet => aria-posinset
+
+    if (!Object.getOwnPropertyDescriptor(Element.prototype, prop)) {
+        Object.defineProperty(Element.prototype, prop, {
+            get() {
+                return this.getAttribute(attribute);
+            },
+            set(value) {
+                // Per the spec, only null is treated as removing the attribute. However, Chromium/WebKit currently
+                // differ from the spec and allow undefined as well. Here, we follow the spec, as well as
+                // our historical behavior. See: https://github.com/w3c/aria/issues/1858
+                if (value === null) {
+                    this.removeAttribute(attribute);
+                } else {
+                    this.setAttribute(attribute, value);
+                }
+            },
+            // configurable and enumerable to allow it to be overridden – this mimics Safari's/Chrome's behavior
+            configurable: true,
+            enumerable: true,
+        });
     }
 }
diff --git a/packages/@lwc/aria-reflection/src/polyfill.ts b/packages/@lwc/aria-reflection/src/polyfill.ts
deleted file mode 100644
index 9b29dea8a9..0000000000
--- a/packages/@lwc/aria-reflection/src/polyfill.ts
+++ /dev/null
@@ -1,34 +0,0 @@
-/*
- * Copyright (c) 2018, salesforce.com, inc.
- * All rights reserved.
- * SPDX-License-Identifier: MIT
- * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
- */
-import { AriaPropNameToAttrNameMap, isNull, defineProperty } from '@lwc/shared';
-
-function createAriaPropertyPropertyDescriptor(attrName: string): PropertyDescriptor {
-    // Note that we need to call this.{get,set,has,remove}Attribute rather than dereferencing
-    // from Element.prototype, because these methods are overridden in LightningElement.
-    return {
-        get(this: HTMLElement): any {
-            // reflect what's in the attribute
-            return this.hasAttribute(attrName) ? this.getAttribute(attrName) : null;
-        },
-        set(this: HTMLElement, newValue: any) {
-            // reflect into the corresponding attribute
-            if (isNull(newValue)) {
-                this.removeAttribute(attrName);
-            } else {
-                this.setAttribute(attrName, newValue);
-            }
-        },
-        configurable: true,
-        enumerable: true,
-    };
-}
-
-export function patch(propName: string, prototype: any) {
-    const attrName = AriaPropNameToAttrNameMap[propName];
-    const descriptor = createAriaPropertyPropertyDescriptor(attrName);
-    defineProperty(prototype, propName, descriptor);
-}
diff --git a/packages/@lwc/engine-core/package.json b/packages/@lwc/engine-core/package.json
index 023faa4657..eda29562d4 100644
--- a/packages/@lwc/engine-core/package.json
+++ b/packages/@lwc/engine-core/package.json
@@ -42,7 +42,6 @@
         }
     },
     "dependencies": {
-        "@lwc/aria-reflection": "3.8.0",
         "@lwc/features": "3.8.0",
         "@lwc/shared": "3.8.0"
     },
diff --git a/packages/@lwc/engine-core/src/framework/base-bridge-element.ts b/packages/@lwc/engine-core/src/framework/base-bridge-element.ts
index c52a869b6e..fb64a6261f 100644
--- a/packages/@lwc/engine-core/src/framework/base-bridge-element.ts
+++ b/packages/@lwc/engine-core/src/framework/base-bridge-element.ts
@@ -23,7 +23,7 @@ import {
     htmlPropertyToAttribute,
     isNull,
 } from '@lwc/shared';
-import { applyAriaReflection } from '@lwc/aria-reflection';
+import { applyAriaReflection } from '../libs/aria-reflection/aria-reflection';
 import { logWarn } from '../shared/logger';
 import { getAssociatedVM } from './vm';
 import { getReadOnlyProxy } from './membrane';
@@ -263,15 +263,15 @@ if (process.env.IS_BROWSER) {
     // This ARIA reflection only really makes sense in the browser. On the server, there is no `renderedCallback()`,
     // so you cannot do e.g. `this.template.querySelector('x-child').ariaBusy = 'true'`. So we don't need to expose
     // ARIA props outside the LightningElement
-    if (lwcRuntimeFlags.DISABLE_ARIA_REFLECTION_POLYFILL) {
-        // If ARIA reflection is not applied globally to Element.prototype, apply it to HTMLBridgeElement.prototype.
-        // This allows `elm.aria*` property accessors to work from outside a component, and to reflect `aria-*` attrs.
-        // This is especially important because the template compiler compiles aria-* attrs on components to aria* props
-        //
-        // Also note that we apply this to BaseBridgeElement.prototype to avoid excessively redefining property
-        // accessors inside the HTMLBridgeElementFactory.
-        applyAriaReflection(BaseBridgeElement.prototype);
-    }
+    //
+    // Apply ARIA reflection to HTMLBridgeElement.prototype. This allows `elm.aria*` property accessors to work from
+    // outside a component, and to reflect `aria-*` attrs. This is especially important because the template compiler
+    // compiles aria-* attrs on components to aria* props.
+    // Note this works regardless of whether the global ARIA reflection polyfill is applied or not.
+    //
+    // Also note that we apply this to BaseBridgeElement.prototype to avoid excessively redefining property
+    // accessors inside the HTMLBridgeElementFactory.
+    applyAriaReflection(BaseBridgeElement.prototype);
 }
 
 freeze(BaseBridgeElement);
diff --git a/packages/@lwc/engine-core/src/framework/base-lightning-element.ts b/packages/@lwc/engine-core/src/framework/base-lightning-element.ts
index 0558e4de05..8c68712b6d 100644
--- a/packages/@lwc/engine-core/src/framework/base-lightning-element.ts
+++ b/packages/@lwc/engine-core/src/framework/base-lightning-element.ts
@@ -28,10 +28,10 @@ import {
     keys,
     setPrototypeOf,
 } from '@lwc/shared';
-import { applyAriaReflection } from '@lwc/aria-reflection';
 
 import { logError, logWarn } from '../shared/logger';
 import { getComponentTag } from '../shared/format';
+import { applyAriaReflection } from '../libs/aria-reflection/aria-reflection';
 
 import { HTMLElementOriginalDescriptors } from './html-properties';
 import { getWrappedComponentsListener } from './component';
@@ -816,16 +816,10 @@ for (const propName in HTMLElementOriginalDescriptors) {
 
 defineProperties(LightningElement.prototype, lightningBasedDescriptors);
 
-function applyAriaReflectionToLightningElement() {
-    // If ARIA reflection is not applied globally to Element.prototype, or if we are running server-side,
-    // apply it to LightningElement.prototype.
-    // This allows `this.aria*` property accessors to work from inside a component, and to reflect `aria-*` attrs.
-    applyAriaReflection(LightningElement.prototype);
-}
-
-if (!process.env.IS_BROWSER || lwcRuntimeFlags.DISABLE_ARIA_REFLECTION_POLYFILL) {
-    applyAriaReflectionToLightningElement();
-}
+// Apply ARIA reflection to LightningElement.prototype, on both the browser and server.
+// This allows `this.aria*` property accessors to work from inside a component, and to reflect `aria-*` attrs.
+// Note this works regardless of whether the global ARIA reflection polyfill is applied or not.
+applyAriaReflection(LightningElement.prototype);
 
 defineProperty(LightningElement, 'CustomElementConstructor', {
     get() {
diff --git a/packages/@lwc/engine-core/src/libs/aria-reflection/aria-reflection.ts b/packages/@lwc/engine-core/src/libs/aria-reflection/aria-reflection.ts
new file mode 100644
index 0000000000..6df29c5026
--- /dev/null
+++ b/packages/@lwc/engine-core/src/libs/aria-reflection/aria-reflection.ts
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2023, salesforce.com, inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: MIT
+ * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
+ */
+import {
+    AriaPropNameToAttrNameMap,
+    defineProperty,
+    getOwnPropertyDescriptor,
+    isNull,
+    isUndefined,
+    keys,
+} from '@lwc/shared';
+import { LightningElement } from '../../framework/base-lightning-element';
+
+// Apply ARIA string reflection behavior to a prototype.
+// This is deliberately kept separate from @lwc/aria-reflection. @lwc/aria-reflection is a global polyfill that is
+// needed for backwards compatibility in LEX, whereas `applyAriaReflection` is designed to only apply to our own
+// LightningElement/BaseBridgeElement prototypes.
+export function applyAriaReflection(prototype: HTMLElement | LightningElement) {
+    for (const propName of keys(AriaPropNameToAttrNameMap)) {
+        const attrName = AriaPropNameToAttrNameMap[propName];
+        if (isUndefined(getOwnPropertyDescriptor(prototype, propName))) {
+            // Note that we need to call this.{get,set,has,remove}Attribute rather than dereferencing
+            // from Element.prototype, because these methods are overridden in LightningElement.
+            defineProperty(prototype, propName, {
+                get(this: HTMLElement): any {
+                    return this.getAttribute(attrName);
+                },
+                set(this: HTMLElement, newValue: any) {
+                    // TODO [#3284]: there is disagreement between browsers and the spec on how to treat undefined
+                    // Our historical behavior is to only treat null as removing the attribute
+                    // See also https://github.com/w3c/aria/issues/1858
+                    if (isNull(newValue)) {
+                        this.removeAttribute(attrName);
+                    } else {
+                        this.setAttribute(attrName, newValue);
+                    }
+                },
+                // configurable and enumerable to allow it to be overridden – this mimics Safari's/Chrome's behavior
+                configurable: true,
+                enumerable: true,
+            });
+        }
+    }
+}
diff --git a/packages/@lwc/engine-core/src/patches/detect-non-standard-aria.ts b/packages/@lwc/engine-core/src/patches/detect-non-standard-aria.ts
index 61be617283..38567d6d23 100644
--- a/packages/@lwc/engine-core/src/patches/detect-non-standard-aria.ts
+++ b/packages/@lwc/engine-core/src/patches/detect-non-standard-aria.ts
@@ -7,7 +7,6 @@
 
 import { defineProperty, getOwnPropertyDescriptor, isNull, isUndefined } from '@lwc/shared';
 import { onReportingEnabled, report, ReportingEventId } from '../framework/reporting';
-import { LightningElement } from '../framework/base-lightning-element';
 import { logWarnOnce } from '../shared/logger';
 import { getAssociatedVMIfPresent, VM } from '../framework/vm';
 import { BaseBridgeElement } from '../framework/base-bridge-element';
@@ -29,10 +28,11 @@ const NON_STANDARD_ARIA_PROPS = [
     'ariaOwns',
 ];
 
-function isLightningElement(elm: Element) {
-    // The former case is for `this.prop` (inside component) and the latter is for `element.prop` (outside component).
-    // In both cases, we apply the non-standard prop even when the global polyfill is disabled, so this is kosher.
-    return elm instanceof LightningElement || elm instanceof BaseBridgeElement;
+function isGlobalAriaPolyfillLoaded(): boolean {
+    // Sniff for the legacy polyfill being loaded. The reason this works is because ariaActiveDescendant is a
+    // non-standard ARIA property reflection that is only supported in our legacy polyfill. See
+    // @lwc/aria-reflection/README.md for details.
+    return !isUndefined(getOwnPropertyDescriptor(Element.prototype, 'ariaActiveDescendant'));
 }
 
 function findVM(elm: Element): VM | undefined {
@@ -45,7 +45,8 @@ function findVM(elm: Element): VM | undefined {
     // Else it might be a light DOM component. Walk up the tree trying to find the owner
     let parentElement: Element | null = elm;
     while (!isNull((parentElement = parentElement.parentElement))) {
-        if (isLightningElement(parentElement)) {
+        if (parentElement instanceof BaseBridgeElement) {
+            // parentElement is an LWC component
             const vm = getAssociatedVMIfPresent(parentElement);
             if (!isUndefined(vm)) {
                 return vm;
@@ -56,32 +57,30 @@ function findVM(elm: Element): VM | undefined {
 }
 
 function checkAndReportViolation(elm: Element, prop: string, isSetter: boolean, setValue: any) {
-    if (!isLightningElement(elm)) {
-        const vm = findVM(elm);
+    const vm = findVM(elm);
 
-        if (process.env.NODE_ENV !== 'production') {
-            logWarnOnce(
-                `Element <${elm.tagName.toLowerCase()}> ` +
-                    (isUndefined(vm) ? '' : `owned by <${vm.elm.tagName.toLowerCase()}> `) +
-                    `uses non-standard property "${prop}". This will be removed in a future version of LWC. ` +
-                    `See https://sfdc.co/deprecated-aria`
-            );
-        }
+    if (process.env.NODE_ENV !== 'production') {
+        logWarnOnce(
+            `Element <${elm.tagName.toLowerCase()}> ` +
+                (isUndefined(vm) ? '' : `owned by <${vm.elm.tagName.toLowerCase()}> `) +
+                `uses non-standard property "${prop}". This will be removed in a future version of LWC. ` +
+                `See https://sfdc.co/deprecated-aria`
+        );
+    }
 
-        let setValueType: string | undefined;
-        if (isSetter) {
-            // `typeof null` is "object" which is not very useful for detecting null.
-            // We mostly want to know null vs undefined vs other types here, due to
-            // https://github.com/salesforce/lwc/issues/3284
-            setValueType = isNull(setValue) ? 'null' : typeof setValue;
-        }
-        report(ReportingEventId.NonStandardAriaReflection, {
-            tagName: vm?.tagName,
-            propertyName: prop,
-            isSetter,
-            setValueType,
-        });
+    let setValueType: string | undefined;
+    if (isSetter) {
+        // `typeof null` is "object" which is not very useful for detecting null.
+        // We mostly want to know null vs undefined vs other types here, due to
+        // https://github.com/salesforce/lwc/issues/3284
+        setValueType = isNull(setValue) ? 'null' : typeof setValue;
     }
+    report(ReportingEventId.NonStandardAriaReflection, {
+        tagName: vm?.tagName,
+        propertyName: prop,
+        isSetter,
+        setValueType,
+    });
 }
 
 function enableDetection() {
@@ -103,6 +102,9 @@ function enableDetection() {
         }
         // @ts-ignore
         const { get, set } = descriptor;
+        // It's important for this defineProperty call to happen _after_ ARIA accessors are applied to the
+        // BaseBridgeElement and LightningElement prototypes. Otherwise, we will log/report for access of non-standard
+        // props on these prototypes, which we actually don't want. We only care about access on generic HTMLElements.
         defineProperty(prototype, prop, {
             get() {
                 checkAndReportViolation(this, prop, false, undefined);
@@ -119,14 +121,12 @@ function enableDetection() {
 }
 
 // No point in running this code if we're not in a browser, or if the global polyfill is not loaded
-if (process.env.IS_BROWSER) {
-    if (!lwcRuntimeFlags.DISABLE_ARIA_REFLECTION_POLYFILL) {
-        // Always run detection in dev mode, so we can at least print to the console
-        if (process.env.NODE_ENV !== 'production') {
-            enableDetection();
-        } else {
-            // In prod mode, only enable detection if reporting is enabled
-            onReportingEnabled(enableDetection);
-        }
+if (process.env.IS_BROWSER && isGlobalAriaPolyfillLoaded()) {
+    // Always run detection in dev mode, so we can at least print to the console
+    if (process.env.NODE_ENV !== 'production') {
+        enableDetection();
+    } else {
+        // In prod mode, only enable detection if reporting is enabled
+        onReportingEnabled(enableDetection);
     }
 }
diff --git a/packages/@lwc/engine-dom/package.json b/packages/@lwc/engine-dom/package.json
index 404b25ae33..1935062349 100644
--- a/packages/@lwc/engine-dom/package.json
+++ b/packages/@lwc/engine-dom/package.json
@@ -42,7 +42,6 @@
         }
     },
     "devDependencies": {
-        "@lwc/aria-reflection": "3.8.0",
         "@lwc/engine-core": "3.8.0",
         "@lwc/shared": "3.8.0"
     },
diff --git a/packages/@lwc/engine-dom/src/aria-reflection-polyfill.ts b/packages/@lwc/engine-dom/src/aria-reflection-polyfill.ts
deleted file mode 100644
index b3aebab5fa..0000000000
--- a/packages/@lwc/engine-dom/src/aria-reflection-polyfill.ts
+++ /dev/null
@@ -1,13 +0,0 @@
-/*
- * Copyright (c) 2018, salesforce.com, inc.
- * All rights reserved.
- * SPDX-License-Identifier: MIT
- * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
- */
-import { applyAriaReflection } from '@lwc/aria-reflection';
-
-if (!lwcRuntimeFlags.DISABLE_ARIA_REFLECTION_POLYFILL) {
-    // If DISABLE_ARIA_REFLECTION_POLYFILL is false, then we need to apply the ARIA reflection polyfill globally,
-    // i.e. to the global Element.prototype
-    applyAriaReflection();
-}
diff --git a/packages/@lwc/engine-dom/src/index.ts b/packages/@lwc/engine-dom/src/index.ts
index de409ce235..bc3b9fcab6 100644
--- a/packages/@lwc/engine-dom/src/index.ts
+++ b/packages/@lwc/engine-dom/src/index.ts
@@ -8,9 +8,6 @@
 // Globals -----------------------------------------------------------------------------------------
 import '@lwc/features';
 
-// Polyfills ---------------------------------------------------------------------------------------
-import './aria-reflection-polyfill';
-
 // DevTools Formatters
 import './formatters';
 
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts b/packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts
index e1de9734c7..b30be8bb89 100755
--- a/packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts
@@ -228,8 +228,4 @@ describe('fixtures', () => {
     describe('native custom element lifecycle', () => {
         testWithFeatureFlagEnabled('ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE');
     });
-
-    describe('disable aria reflection polyfill', () => {
-        testWithFeatureFlagEnabled('DISABLE_ARIA_REFLECTION_POLYFILL');
-    });
 });
diff --git a/packages/@lwc/features/src/index.ts b/packages/@lwc/features/src/index.ts
index 1ed2dc5735..f362292df6 100644
--- a/packages/@lwc/features/src/index.ts
+++ b/packages/@lwc/features/src/index.ts
@@ -17,7 +17,6 @@ const features: FeatureFlagMap = {
     ENABLE_WIRE_SYNC_EMIT: null,
     DISABLE_LIGHT_DOM_UNSCOPED_CSS: null,
     ENABLE_FROZEN_TEMPLATE: null,
-    DISABLE_ARIA_REFLECTION_POLYFILL: null,
     ENABLE_LEGACY_SCOPE_TOKENS: null,
 };
 
diff --git a/packages/@lwc/features/src/types.ts b/packages/@lwc/features/src/types.ts
index 3779c1ab6a..46185538ec 100644
--- a/packages/@lwc/features/src/types.ts
+++ b/packages/@lwc/features/src/types.ts
@@ -60,13 +60,6 @@ export interface FeatureFlagMap {
      */
     ENABLE_FROZEN_TEMPLATE: FeatureFlagValue;
 
-    /**
-     * Flag to remove the ARIA reflection polyfill. When set to true, this flag will avoid the global DOM patching
-     * to polyfill ARIA reflection. Instead, the necessary ARIA properties will only exist on the LightningElement
-     * and HTMLBridgeElement base classes, not on every Element.
-     */
-    DISABLE_ARIA_REFLECTION_POLYFILL: FeatureFlagValue;
-
     /**
      * If true, render legacy CSS scope tokens in addition to the modern CSS scope tokens. This is designed
      * for cases where backwards compat is required (e.g. global stylesheets using these tokens in their selectors).
diff --git a/packages/@lwc/integration-karma/README.md b/packages/@lwc/integration-karma/README.md
index f637f65def..491a3b669a 100644
--- a/packages/@lwc/integration-karma/README.md
+++ b/packages/@lwc/integration-karma/README.md
@@ -34,7 +34,7 @@ This set of environment variables applies to the `start` and `test` commands:
 -   **`DISABLE_SYNTHETIC=1`:** Run without any synthetic shadow polyfill patches.
 -   **`FORCE_NATIVE_SHADOW_MODE_FOR_TEST=1`:** Force tests to run in native shadow mode with synthetic shadow polyfill patches.
 -   **`ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE=1`:** Use native custom element lifecycle callbacks.
--   **`DISABLE_ARIA_REFLECTION_POLYFILL=1`:** Disable usage of `@lwc/aria-reflection` as a global polyfill.
+-   **`ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL=1`:** ARIA string reflection as a global polyfill.
 -   **`NODE_ENV_FOR_TEST`**: Set the `NODE_ENV` to be used for the tests (at runtime, in the browser).
 -   **`COVERAGE=1`:** Gather engine code coverage, and store it in the `coverage` folder.
 -   **`GREP="pattern"`:** Filter the spec to run based on the pattern.
diff --git a/packages/@lwc/integration-karma/scripts/karma-configs/test/base.js b/packages/@lwc/integration-karma/scripts/karma-configs/test/base.js
index d2782749c1..5f437ad310 100644
--- a/packages/@lwc/integration-karma/scripts/karma-configs/test/base.js
+++ b/packages/@lwc/integration-karma/scripts/karma-configs/test/base.js
@@ -12,7 +12,12 @@ const path = require('path');
 const karmaPluginLwc = require('../../karma-plugins/lwc');
 const karmaPluginEnv = require('../../karma-plugins/env');
 const karmaPluginTransformFramework = require('../../karma-plugins/transform-framework.js');
-const { SYNTHETIC_SHADOW_ENABLED, GREP, COVERAGE } = require('../../shared/options');
+const {
+    SYNTHETIC_SHADOW_ENABLED,
+    GREP,
+    COVERAGE,
+    ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL,
+} = require('../../shared/options');
 const { createPattern } = require('../utils');
 const TAGS = require('./tags');
 
@@ -22,11 +27,12 @@ const COVERAGE_DIR = path.resolve(__dirname, '../../../coverage');
 const SYNTHETIC_SHADOW = require.resolve('@lwc/synthetic-shadow/dist/index.js');
 const LWC_ENGINE = require.resolve('@lwc/engine-dom/dist/index.js');
 const WIRE_SERVICE = require.resolve('@lwc/wire-service/dist/index.js');
+const ARIA_REFLECTION = require.resolve('@lwc/aria-reflection/dist/index.js');
 
 const TEST_UTILS = require.resolve('../../../helpers/test-utils');
 const TEST_SETUP = require.resolve('../../../helpers/test-setup');
 
-const ALL_FRAMEWORK_FILES = [SYNTHETIC_SHADOW, LWC_ENGINE, WIRE_SERVICE];
+const ALL_FRAMEWORK_FILES = [SYNTHETIC_SHADOW, LWC_ENGINE, WIRE_SERVICE, ARIA_REFLECTION];
 
 // Fix Node warning about >10 event listeners ("Possible EventEmitter memory leak detected").
 // This is due to the fact that we are running so many simultaneous rollup commands
@@ -40,6 +46,9 @@ function getFiles() {
     if (SYNTHETIC_SHADOW_ENABLED) {
         frameworkFiles.push(createPattern(SYNTHETIC_SHADOW));
     }
+    if (ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL) {
+        frameworkFiles.push(createPattern(ARIA_REFLECTION));
+    }
     frameworkFiles.push(createPattern(LWC_ENGINE));
     frameworkFiles.push(createPattern(WIRE_SERVICE));
     frameworkFiles.push(createPattern(TEST_SETUP));
diff --git a/packages/@lwc/integration-karma/scripts/karma-configs/test/tags.js b/packages/@lwc/integration-karma/scripts/karma-configs/test/tags.js
index 0bad54f970..7dd342f841 100644
--- a/packages/@lwc/integration-karma/scripts/karma-configs/test/tags.js
+++ b/packages/@lwc/integration-karma/scripts/karma-configs/test/tags.js
@@ -12,7 +12,7 @@ const {
     SYNTHETIC_SHADOW_ENABLED,
     FORCE_NATIVE_SHADOW_MODE_FOR_TEST,
     ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE,
-    DISABLE_ARIA_REFLECTION_POLYFILL,
+    ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL,
     DISABLE_SYNTHETIC_SHADOW_SUPPORT_IN_COMPILER,
     NODE_ENV_FOR_TEST,
     API_VERSION,
@@ -24,7 +24,7 @@ const TAGS = [
     FORCE_NATIVE_SHADOW_MODE_FOR_TEST && 'force-native-shadow-mode',
     LEGACY_BROWSERS && 'legacy-browsers',
     ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE && 'native-lifecycle',
-    DISABLE_ARIA_REFLECTION_POLYFILL && 'disable-aria-polyfill',
+    ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL && 'aria-polyfill',
     DISABLE_SYNTHETIC_SHADOW_SUPPORT_IN_COMPILER && 'disable-synthetic-in-compiler',
     `NODE_ENV-${NODE_ENV_FOR_TEST}`,
     API_VERSION && `api-version-${API_VERSION}`,
diff --git a/packages/@lwc/integration-karma/scripts/karma-plugins/env.js b/packages/@lwc/integration-karma/scripts/karma-plugins/env.js
index 37ea2febf2..e5f3866286 100644
--- a/packages/@lwc/integration-karma/scripts/karma-plugins/env.js
+++ b/packages/@lwc/integration-karma/scripts/karma-plugins/env.js
@@ -19,7 +19,7 @@ const {
     FORCE_NATIVE_SHADOW_MODE_FOR_TEST,
     SYNTHETIC_SHADOW_ENABLED,
     ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE,
-    DISABLE_ARIA_REFLECTION_POLYFILL,
+    ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL,
     NODE_ENV_FOR_TEST,
     API_VERSION,
 } = require('../shared/options');
@@ -37,8 +37,7 @@ function createEnvFile() {
         `
         window.lwcRuntimeFlags = {
             ENABLE_FORCE_NATIVE_SHADOW_MODE_FOR_TEST: ${FORCE_NATIVE_SHADOW_MODE_FOR_TEST},
-            ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE: ${ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE},
-            DISABLE_ARIA_REFLECTION_POLYFILL: ${DISABLE_ARIA_REFLECTION_POLYFILL}
+            ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE: ${ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE}
         };
         window.process = {
             env: {
@@ -47,6 +46,7 @@ function createEnvFile() {
                 NATIVE_SHADOW: ${!SYNTHETIC_SHADOW_ENABLED || FORCE_NATIVE_SHADOW_MODE_FOR_TEST},
                 NATIVE_SHADOW_ROOT_DEFINED: typeof ShadowRoot !== 'undefined',
                 SYNTHETIC_SHADOW_ENABLED: ${SYNTHETIC_SHADOW_ENABLED},
+                ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL: ${ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL},
                 LWC_VERSION: ${JSON.stringify(LWC_VERSION)},
                 API_VERSION: ${JSON.stringify(API_VERSION)}
             }
diff --git a/packages/@lwc/integration-karma/scripts/karma-plugins/transform-framework.js b/packages/@lwc/integration-karma/scripts/karma-plugins/transform-framework.js
index 54948cf1fe..68e3dea130 100644
--- a/packages/@lwc/integration-karma/scripts/karma-plugins/transform-framework.js
+++ b/packages/@lwc/integration-karma/scripts/karma-plugins/transform-framework.js
@@ -24,6 +24,9 @@ function getIifeName(filename) {
     } else if (filename.includes('@lwc/synthetic-shadow')) {
         // synthetic shadow does not need an IIFE name
         return undefined;
+    } else if (filename.includes('aria-reflection')) {
+        // aria reflection global polyfill does not need an IIFE name
+        return undefined;
     }
     throw new Error(`Unknown framework filename, not sure which IIFE name to use: ${filename}`);
 }
diff --git a/packages/@lwc/integration-karma/scripts/shared/options.js b/packages/@lwc/integration-karma/scripts/shared/options.js
index dede4cee9b..c85331aa2b 100644
--- a/packages/@lwc/integration-karma/scripts/shared/options.js
+++ b/packages/@lwc/integration-karma/scripts/shared/options.js
@@ -18,7 +18,9 @@ const FORCE_NATIVE_SHADOW_MODE_FOR_TEST = Boolean(process.env.FORCE_NATIVE_SHADO
 const ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE = Boolean(
     process.env.ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE
 );
-const DISABLE_ARIA_REFLECTION_POLYFILL = Boolean(process.env.DISABLE_ARIA_REFLECTION_POLYFILL);
+const ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL = Boolean(
+    process.env.ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL
+);
 const DISABLE_SYNTHETIC_SHADOW_SUPPORT_IN_COMPILER = Boolean(
     process.env.DISABLE_SYNTHETIC_SHADOW_SUPPORT_IN_COMPILER
 );
@@ -29,7 +31,7 @@ module.exports = {
     // Test configuration
     LEGACY_BROWSERS,
     ENABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE,
-    DISABLE_ARIA_REFLECTION_POLYFILL,
+    ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL,
     NODE_ENV_FOR_TEST,
     FORCE_NATIVE_SHADOW_MODE_FOR_TEST,
     DISABLE_SYNTHETIC_SHADOW_SUPPORT_IN_COMPILER,
diff --git a/packages/@lwc/integration-karma/test/accessibility/non-standard-aria-props/index.spec.js b/packages/@lwc/integration-karma/test/accessibility/non-standard-aria-props/index.spec.js
index fbe8e84f9d..b300972e27 100644
--- a/packages/@lwc/integration-karma/test/accessibility/non-standard-aria-props/index.spec.js
+++ b/packages/@lwc/integration-karma/test/accessibility/non-standard-aria-props/index.spec.js
@@ -4,7 +4,7 @@ import Light from 'x/light';
 import Shadow from 'x/shadow';
 
 // This test only works if the ARIA reflection polyfill is loaded
-if (!window.lwcRuntimeFlags.DISABLE_ARIA_REFLECTION_POLYFILL) {
+if (process.env.ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL) {
     describe('non-standard ARIA properties', () => {
         let dispatcher;
 
diff --git a/packages/@lwc/integration-karma/test/accessibility/synthetic-cross-root-aria/index.spec.js b/packages/@lwc/integration-karma/test/accessibility/synthetic-cross-root-aria/index.spec.js
index ed88b866a0..2ae25de457 100644
--- a/packages/@lwc/integration-karma/test/accessibility/synthetic-cross-root-aria/index.spec.js
+++ b/packages/@lwc/integration-karma/test/accessibility/synthetic-cross-root-aria/index.spec.js
@@ -37,7 +37,15 @@ if (!process.env.NATIVE_SHADOW) {
                 targetElm = elm.shadowRoot.querySelector('x-aria-target');
             });
 
-            [false, true].forEach((usePropertyAccess) => {
+            const usePropertyAccessValues = [false];
+
+            // It doesn't make sense to test setting e.g. `elm.ariaLabelledBy` if the global
+            // polyfill is not applied
+            if (process.env.ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL) {
+                usePropertyAccessValues.push(true);
+            }
+
+            usePropertyAccessValues.forEach((usePropertyAccess) => {
                 const expectedMessages = [
                     ...(usePropertyAccess ? [expectedMessageForNonStandardAria] : []),
                     expectedMessageForCrossRoot,
diff --git a/packages/@lwc/integration-karma/test/polyfills/aria-properties/index.spec.js b/packages/@lwc/integration-karma/test/polyfills/aria-properties/index.spec.js
index ae5c5f10e2..3f168ab557 100644
--- a/packages/@lwc/integration-karma/test/polyfills/aria-properties/index.spec.js
+++ b/packages/@lwc/integration-karma/test/polyfills/aria-properties/index.spec.js
@@ -1,5 +1,6 @@
 import { ariaPropertiesMapping, nonStandardAriaProperties } from 'test-utils';
-import { __unstable__ReportingControl as reportingControl } from 'lwc';
+import { __unstable__ReportingControl as reportingControl, createElement } from 'lwc';
+import Component from 'x/component';
 
 function testAriaProperty(property, attribute) {
     describe(property, () => {
@@ -189,8 +190,45 @@ function testAriaProperty(property, attribute) {
 }
 
 // These tests don't make sense if the global polyfill is not loaded
-if (!window.lwcRuntimeFlags.DISABLE_ARIA_REFLECTION_POLYFILL) {
+if (process.env.ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL) {
     for (const [ariaProperty, ariaAttribute] of Object.entries(ariaPropertiesMapping)) {
         testAriaProperty(ariaProperty, ariaAttribute);
     }
+
+    describe('non-standard properties do not log/report for LightningElement/BaseBridgeElement', () => {
+        let dispatcher;
+
+        beforeEach(() => {
+            dispatcher = jasmine.createSpy();
+            reportingControl.attachDispatcher(dispatcher);
+        });
+
+        afterEach(() => {
+            reportingControl.detachDispatcher();
+        });
+
+        nonStandardAriaProperties.forEach((prop) => {
+            describe(prop, () => {
+                it('LightningElement (internal)', () => {
+                    const elm = createElement('x-component', { is: Component });
+                    document.body.appendChild(elm);
+
+                    expect(() => {
+                        elm.getProp(prop);
+                    }).not.toLogWarningDev();
+                    expect(dispatcher).not.toHaveBeenCalled();
+                });
+
+                it('BaseBridgeElement (external)', () => {
+                    const elm = createElement('x-component', { is: Component });
+                    document.body.appendChild(elm);
+
+                    expect(() => {
+                        elm[prop] = 'foo';
+                    }).not.toLogWarningDev();
+                    expect(dispatcher).not.toHaveBeenCalled();
+                });
+            });
+        });
+    });
 }
diff --git a/packages/@lwc/integration-karma/test/polyfills/aria-properties/x/component/component.html b/packages/@lwc/integration-karma/test/polyfills/aria-properties/x/component/component.html
new file mode 100644
index 0000000000..cc340bc4c9
--- /dev/null
+++ b/packages/@lwc/integration-karma/test/polyfills/aria-properties/x/component/component.html
@@ -0,0 +1 @@
+<template></template>
diff --git a/packages/@lwc/integration-karma/test/polyfills/aria-properties/x/component/component.js b/packages/@lwc/integration-karma/test/polyfills/aria-properties/x/component/component.js
new file mode 100644
index 0000000000..a9afef90cd
--- /dev/null
+++ b/packages/@lwc/integration-karma/test/polyfills/aria-properties/x/component/component.js
@@ -0,0 +1,8 @@
+import { LightningElement, api } from 'lwc';
+
+export default class extends LightningElement {
+    @api
+    getProp(name) {
+        return this[name];
+    }
+}
diff --git a/packages/@lwc/integration-karma/test/template/attribute-aria/index.spec.js b/packages/@lwc/integration-karma/test/template/attribute-aria/index.spec.js
index 99b501e4f2..a8d338a36a 100644
--- a/packages/@lwc/integration-karma/test/template/attribute-aria/index.spec.js
+++ b/packages/@lwc/integration-karma/test/template/attribute-aria/index.spec.js
@@ -74,7 +74,7 @@ describe('setting aria attributes', () => {
 
         // If the polyfill is not enabled, then we can't expect the div to have all the ARIA properties,
         // because some are non-standard or not supported in all browsers
-        if (!window.lwcRuntimeFlags.DISABLE_ARIA_REFLECTION_POLYFILL) {
+        if (process.env.ENABLE_ARIA_REFLECTION_GLOBAL_POLYFILL) {
             it('aria prop is set', () => {
                 for (const prop of ariaProperties) {
                     expect(childComponent[prop]).toMatch(/^foo/);