diff --git a/src/jquery/css.js b/src/jquery/css.js
index fadcb11d..a2cc84ae 100644
--- a/src/jquery/css.js
+++ b/src/jquery/css.js
@@ -2,7 +2,7 @@ import { jQueryVersionSince } from "../compareVersions.js";
 import { migrateWarn, migratePatchFunc } from "../main.js";
 import { camelCase } from "../utils.js";
 
-var origFnCss,
+var origFnCss, internalCssNumber,
 	internalSwapCall = false,
 	ralphaStart = /^[a-z]/,
 
@@ -84,8 +84,11 @@ if ( jQueryVersionSince( "3.4.0" ) && typeof Proxy !== "undefined" ) {
 // https://github.com/jquery/jquery/blob/3.6.0/src/css.js#L212-L233
 // This way, number values for the CSS properties below won't start triggering
 // Migrate warnings when jQuery gets updated to >=4.0.0 (gh-438).
-if ( jQueryVersionSince( "4.0.0" ) && typeof Proxy !== "undefined" ) {
-	jQuery.cssNumber = new Proxy( {
+if ( jQueryVersionSince( "4.0.0" ) ) {
+
+	// We need to keep this as a local variable as we need it internally
+	// in a `jQuery.fn.css` patch and this usage shouldn't warn.
+	internalCssNumber = {
 		animationIterationCount: true,
 		columnCount: true,
 		fillOpacity: true,
@@ -106,16 +109,31 @@ if ( jQueryVersionSince( "4.0.0" ) && typeof Proxy !== "undefined" ) {
 		widows: true,
 		zIndex: true,
 		zoom: true
-	}, {
-		get: function() {
-			migrateWarn( "css-number", "jQuery.cssNumber is deprecated" );
-			return Reflect.get.apply( this, arguments );
-		},
-		set: function() {
-			migrateWarn( "css-number", "jQuery.cssNumber is deprecated" );
-			return Reflect.set.apply( this, arguments );
-		}
-	} );
+	};
+
+	if ( typeof Proxy !== "undefined" ) {
+		jQuery.cssNumber = new Proxy( internalCssNumber, {
+			get: function() {
+				migrateWarn( "css-number", "jQuery.cssNumber is deprecated" );
+				return Reflect.get.apply( this, arguments );
+			},
+			set: function() {
+				migrateWarn( "css-number", "jQuery.cssNumber is deprecated" );
+				return Reflect.set.apply( this, arguments );
+			}
+		} );
+	} else {
+
+		// Support: IE 9-11+
+		// IE doesn't support proxies, but we still want to restore the legacy
+		// jQuery.cssNumber there.
+		jQuery.cssNumber = internalCssNumber;
+	}
+} else {
+
+	// Make `internalCssNumber` defined for jQuery <4 as well as it's needed
+	// in the `jQuery.fn.css` patch below.
+	internalCssNumber = jQuery.cssNumber;
 }
 
 function isAutoPx( prop ) {
@@ -142,7 +160,10 @@ migratePatchFunc( jQuery.fn, "css", function( name, value ) {
 
 	if ( typeof value === "number" ) {
 		camelName = camelCase( name );
-		if ( !isAutoPx( camelName ) && !jQuery.cssNumber[ camelName ] ) {
+
+		// Use `internalCssNumber` to avoid triggering our warnings in this
+		// internal check.
+		if ( !isAutoPx( camelName ) && !internalCssNumber[ camelName ] ) {
 			migrateWarn( "css-number",
 				"Number-typed values are deprecated for jQuery.fn.css( \"" +
 				name + "\", value )" );