diff --git a/src/jquery/css.js b/src/jquery/css.js
index 77aa10b6..f04b3649 100644
--- a/src/jquery/css.js
+++ b/src/jquery/css.js
@@ -81,10 +81,33 @@ if ( jQueryVersionSince( "3.4.0" ) && typeof Proxy !== "undefined" ) {
 	} );
 }
 
-// Create a dummy jQuery.cssNumber if missing. It won't be used by jQuery but
-// it will prevent code adding new keys to it unconditionally from crashing.
+// In jQuery >=4 where jQuery.cssNumber is missing fill it with the latest 3.x version:
+// 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 ( !jQuery.cssNumber ) {
-	jQuery.cssNumber = {};
+	jQuery.cssNumber = {
+		animationIterationCount: true,
+		columnCount: true,
+		fillOpacity: true,
+		flexGrow: true,
+		flexShrink: true,
+		fontWeight: true,
+		gridArea: true,
+		gridColumn: true,
+		gridColumnEnd: true,
+		gridColumnStart: true,
+		gridRow: true,
+		gridRowEnd: true,
+		gridRowStart: true,
+		lineHeight: true,
+		opacity: true,
+		order: true,
+		orphans: true,
+		widows: true,
+		zIndex: true,
+		zoom: true
+	};
 }
 
 function isAutoPx( prop ) {
diff --git a/test/css.js b/test/css.js
index 4d2c1415..c8627fd7 100644
--- a/test/css.js
+++ b/test/css.js
@@ -62,7 +62,7 @@ QUnit.test( "jQuery.css with arrays", function( assert ) {
 
 QUnit.test( "jQuery.css with numbers", function( assert ) {
 	var jQuery3OrOlder = compareVersions( jQuery.fn.jquery, "4.0.0" ) < 0,
-		whitelist = [
+		allowlist = [
 			"margin",
 			"marginTop",
 			"marginRight",
@@ -95,7 +95,7 @@ QUnit.test( "jQuery.css with numbers", function( assert ) {
 			"borderLeftWidth"
 		];
 
-	assert.expect( jQuery3OrOlder ?  7 : 6 );
+	assert.expect( jQuery3OrOlder ?  8 : 7 );
 
 	function kebabCase( string ) {
 		return string.replace( /[A-Z]/g, function( match ) {
@@ -127,8 +127,8 @@ QUnit.test( "jQuery.css with numbers", function( assert ) {
 		} );
 	} );
 
-	expectNoWarning( assert, "Number value (whitelisted props)", function() {
-		whitelist.forEach( function( prop ) {
+	expectNoWarning( assert, "Number value (allowlisted props)", function() {
+		allowlist.forEach( function( prop ) {
 			jQuery( "<div />" ).css( prop, 1 );
 			jQuery( "<div />" ).css( kebabCase( prop ), 1 );
 		} );
@@ -147,6 +147,14 @@ QUnit.test( "jQuery.css with numbers", function( assert ) {
 		}
 	} );
 
+	// z-index is tested explicitly as raw jQuery 4.0 will not have `jQuery.cssNumber`
+	// so iterating over it won't find anything and we'd like to ensure number values
+	// are not warned against for safe CSS props like z-index (gh-438).
+	expectNoWarning( assert, "z-index", function() {
+		jQuery( "<div />" ).css( "z-index", 1 );
+		jQuery( "<div />" ).css( kebabCase( "zIndex" ), 1 );
+	} );
+
 } );
 
 QUnit.test( "jQuery.cssNumber", function( assert ) {