diff --git a/packages/@lwc/ssr-client-utils/src/__tests__/index.spec.ts b/packages/@lwc/ssr-client-utils/src/__tests__/index.spec.ts
new file mode 100644
index 0000000000..7487c5ed61
--- /dev/null
+++ b/packages/@lwc/ssr-client-utils/src/__tests__/index.spec.ts
@@ -0,0 +1,364 @@
+/*
+ * Copyright (c) 2025, 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 { describe, it, expect, beforeEach, vi } from 'vitest';
+
+// Mock CSS API for testing
+const mockCSSStyleSheet = vi.fn();
+global.CSSStyleSheet = mockCSSStyleSheet;
+
+// Mock customElements API
+const mockDefine = vi.fn();
+global.customElements = {
+    define: mockDefine,
+} as any;
+
+// Mock document.createElement
+global.document = {
+    createElement: vi.fn(),
+} as any;
+
+describe('registerLwcStyleComponent', () => {
+    beforeEach(() => {
+        vi.clearAllMocks();
+    });
+
+    it('should register lwc-style custom element', async () => {
+        const { registerLwcStyleComponent } = await import('../index');
+        registerLwcStyleComponent();
+        expect(mockDefine).toHaveBeenCalledWith('lwc-style', expect.any(Function));
+    });
+});
+
+describe('StyleDeduplicator (lwc-style element)', () => {
+    let StyleDeduplicatorClass: any;
+
+    beforeEach(async () => {
+        vi.clearAllMocks();
+        mockDefine.mockClear();
+        mockCSSStyleSheet.mockClear();
+
+        // Clear modules to get fresh import
+        vi.resetModules();
+
+        // Set up test environment for cache clearing
+        const originalProcess = (global as any).process;
+        (global as any).process = { env: { NODE_ENV: 'test-karma-lwc' } };
+        (global as any).window = {};
+
+        // Import fresh module
+        const { registerLwcStyleComponent } = await import('../index');
+        registerLwcStyleComponent();
+
+        // Get the StyleDeduplicator class
+        StyleDeduplicatorClass = mockDefine.mock.calls[0][1];
+
+        // Clear the cache
+        if ((global as any).window?.__lwcClearStylesheetCache) {
+            (global as any).window.__lwcClearStylesheetCache();
+        }
+
+        // Restore process
+        (global as any).process = originalProcess;
+    });
+
+    describe('connectedCallback', () => {
+        it('should throw error when style-id attribute is missing', () => {
+            const mockElement = {
+                getAttribute: vi.fn().mockReturnValue(null),
+            };
+
+            expect(() => {
+                StyleDeduplicatorClass.prototype.connectedCallback.call(mockElement);
+            }).toThrow('"style-id" attribute must be supplied for <lwc-style> element');
+        });
+
+        it('should throw error when style-id attribute is empty string', () => {
+            const mockElement = {
+                getAttribute: vi.fn().mockReturnValue(''),
+            };
+
+            expect(() => {
+                StyleDeduplicatorClass.prototype.connectedCallback.call(mockElement);
+            }).toThrow('"style-id" attribute must be supplied for <lwc-style> element');
+        });
+
+        it('should throw error when referenced style element does not exist', () => {
+            const mockElement = {
+                getAttribute: vi.fn().mockReturnValue('test-style-id'),
+                getRootNode: vi.fn().mockReturnValue({
+                    getElementById: vi.fn().mockReturnValue(null),
+                }),
+            };
+
+            expect(() => {
+                StyleDeduplicatorClass.prototype.connectedCallback.call(mockElement);
+            }).toThrow(
+                '<lwc-style> tag found with no corresponding <style id="test-style-id"> tag'
+            );
+        });
+
+        it('should create new stylesheet and remove element on first encounter', () => {
+            const mockStylesheet = { replaceSync: vi.fn() };
+            mockCSSStyleSheet.mockReturnValue(mockStylesheet);
+
+            const mockElement = {
+                getAttribute: vi.fn().mockReturnValue('test-style-id'),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue({ innerHTML: 'body { color: red; }' }),
+                }),
+                remove: vi.fn(),
+            };
+
+            StyleDeduplicatorClass.prototype.connectedCallback.call(mockElement);
+
+            expect(mockCSSStyleSheet).toHaveBeenCalled();
+            expect(mockStylesheet.replaceSync).toHaveBeenCalledWith('body { color: red; }');
+            expect(mockElement.remove).toHaveBeenCalled();
+        });
+
+        it('should reuse cached stylesheet and create placeholder on subsequent encounters', () => {
+            const mockStylesheet = { replaceSync: vi.fn() };
+            mockCSSStyleSheet.mockReturnValue(mockStylesheet);
+            const mockPlaceholder = {
+                setAttribute: vi.fn(),
+                classList: { add: vi.fn() },
+            };
+            (document.createElement as any).mockReturnValue(mockPlaceholder);
+
+            const styleId = 'test-style-id';
+            const mockStyleElement = { innerHTML: 'body { color: red; }' };
+
+            // First element - creates cache
+            const firstElement = {
+                getAttribute: vi.fn().mockReturnValue(styleId),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue(mockStyleElement),
+                }),
+                remove: vi.fn(),
+            };
+            StyleDeduplicatorClass.prototype.connectedCallback.call(firstElement);
+
+            // Second element - reuses cache
+            const secondElement = {
+                getAttribute: vi.fn().mockReturnValue(styleId),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue(mockStyleElement),
+                }),
+                classList: {
+                    forEach: vi.fn((callback) => {
+                        callback('scope-class-1');
+                        callback('scope-class-2');
+                    }),
+                },
+                replaceWith: vi.fn(),
+                remove: vi.fn(),
+            };
+
+            StyleDeduplicatorClass.prototype.connectedCallback.call(secondElement);
+
+            // Verify behavior
+            expect(document.createElement).toHaveBeenCalledWith('style');
+            expect(mockPlaceholder.setAttribute).toHaveBeenCalledWith('type', 'text/css');
+            expect(mockPlaceholder.classList.add).toHaveBeenCalledWith('scope-class-1');
+            expect(mockPlaceholder.classList.add).toHaveBeenCalledWith('scope-class-2');
+            expect(secondElement.replaceWith).toHaveBeenCalledWith(mockPlaceholder);
+            expect(secondElement.remove).not.toHaveBeenCalled();
+        });
+
+        it('should handle multiple different style-ids correctly', () => {
+            const mockStylesheet1 = { replaceSync: vi.fn() };
+            const mockStylesheet2 = { replaceSync: vi.fn() };
+            mockCSSStyleSheet
+                .mockReturnValueOnce(mockStylesheet1)
+                .mockReturnValueOnce(mockStylesheet2);
+
+            // First style ID
+            const element1 = {
+                getAttribute: vi.fn().mockReturnValue('style-1'),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue({ innerHTML: 'body { color: red; }' }),
+                }),
+                remove: vi.fn(),
+            };
+            StyleDeduplicatorClass.prototype.connectedCallback.call(element1);
+
+            // Different style ID
+            const element2 = {
+                getAttribute: vi.fn().mockReturnValue('style-2'),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue({ innerHTML: 'body { color: blue; }' }),
+                }),
+                remove: vi.fn(),
+            };
+            StyleDeduplicatorClass.prototype.connectedCallback.call(element2);
+
+            expect(mockStylesheet1.replaceSync).toHaveBeenCalledWith('body { color: red; }');
+            expect(mockStylesheet2.replaceSync).toHaveBeenCalledWith('body { color: blue; }');
+            expect(element1.remove).toHaveBeenCalled();
+            expect(element2.remove).toHaveBeenCalled();
+        });
+
+        it('should preserve class list when creating placeholder element', () => {
+            const mockStylesheet = { replaceSync: vi.fn() };
+            mockCSSStyleSheet.mockReturnValue(mockStylesheet);
+            const mockPlaceholder = {
+                setAttribute: vi.fn(),
+                classList: { add: vi.fn() },
+            };
+            (document.createElement as any).mockReturnValue(mockPlaceholder);
+
+            const styleId = 'test-style-id';
+
+            // First element - creates cache
+            const firstElement = {
+                getAttribute: vi.fn().mockReturnValue(styleId),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue({ innerHTML: 'body { color: red; }' }),
+                }),
+                remove: vi.fn(),
+            };
+            StyleDeduplicatorClass.prototype.connectedCallback.call(firstElement);
+
+            // Second element - should copy classes
+            const classNames = ['lwc-abc123', 'scope-token', 'another-class'];
+            const secondElement = {
+                getAttribute: vi.fn().mockReturnValue(styleId),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue({ innerHTML: 'body { color: red; }' }),
+                }),
+                classList: {
+                    forEach: vi.fn((callback) => {
+                        classNames.forEach(callback);
+                    }),
+                },
+                replaceWith: vi.fn(),
+            };
+
+            StyleDeduplicatorClass.prototype.connectedCallback.call(secondElement);
+
+            classNames.forEach((className) => {
+                expect(mockPlaceholder.classList.add).toHaveBeenCalledWith(className);
+            });
+        });
+
+        it('should handle empty class list without errors', () => {
+            const mockStylesheet = { replaceSync: vi.fn() };
+            mockCSSStyleSheet.mockReturnValue(mockStylesheet);
+            const mockPlaceholder = {
+                setAttribute: vi.fn(),
+                classList: { add: vi.fn() },
+            };
+            (document.createElement as any).mockReturnValue(mockPlaceholder);
+
+            const styleId = 'test-style-id';
+
+            // First element - creates cache
+            const firstElement = {
+                getAttribute: vi.fn().mockReturnValue(styleId),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue({ innerHTML: 'body { color: red; }' }),
+                }),
+                remove: vi.fn(),
+            };
+            StyleDeduplicatorClass.prototype.connectedCallback.call(firstElement);
+
+            // Second element with empty class list
+            const secondElement = {
+                getAttribute: vi.fn().mockReturnValue(styleId),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue({ innerHTML: 'body { color: red; }' }),
+                }),
+                classList: {
+                    forEach: vi.fn(), // Called but no classes to iterate
+                },
+                replaceWith: vi.fn(),
+            };
+
+            expect(() => {
+                StyleDeduplicatorClass.prototype.connectedCallback.call(secondElement);
+            }).not.toThrow();
+
+            expect(secondElement.replaceWith).toHaveBeenCalledWith(mockPlaceholder);
+            expect(mockPlaceholder.classList.add).not.toHaveBeenCalled();
+        });
+
+        it('should add stylesheet to adoptedStyleSheets for cached entries', () => {
+            const mockStylesheet = { replaceSync: vi.fn() };
+            mockCSSStyleSheet.mockReturnValue(mockStylesheet);
+            const styleId = 'test-style-id';
+
+            // First element - creates cache
+            const firstElement = {
+                getAttribute: vi.fn().mockReturnValue(styleId),
+                getRootNode: vi.fn().mockReturnValue({
+                    adoptedStyleSheets: [],
+                    getElementById: vi.fn().mockReturnValue({ innerHTML: 'body { color: red; }' }),
+                }),
+                remove: vi.fn(),
+            };
+            StyleDeduplicatorClass.prototype.connectedCallback.call(firstElement);
+
+            // Second element - should add to adoptedStyleSheets
+            const secondRoot = { adoptedStyleSheets: [] };
+            const secondElement = {
+                getAttribute: vi.fn().mockReturnValue(styleId),
+                getRootNode: vi.fn().mockReturnValue(secondRoot),
+                classList: { forEach: vi.fn() },
+                replaceWith: vi.fn(),
+            };
+
+            const mockPlaceholder = {
+                setAttribute: vi.fn(),
+                classList: { add: vi.fn() },
+            };
+            (document.createElement as any).mockReturnValue(mockPlaceholder);
+
+            StyleDeduplicatorClass.prototype.connectedCallback.call(secondElement);
+
+            expect(secondRoot.adoptedStyleSheets).toHaveLength(1);
+            expect(secondRoot.adoptedStyleSheets[0]).toHaveProperty('replaceSync');
+        });
+    });
+});
+
+describe('Karma test environment', () => {
+    it('should expose cache clearing function in test-karma-lwc environment', async () => {
+        // Save original values
+        const originalProcess = (global as any).process;
+        const originalWindow = (global as any).window;
+
+        try {
+            // Mock test-karma-lwc environment
+            (global as any).process = {
+                env: { NODE_ENV: 'test-karma-lwc' },
+            };
+            (global as any).window = {};
+
+            // Re-import the module to trigger the environment check
+            vi.resetModules();
+
+            // Import should set up the cache clearing function
+            await import('../index');
+
+            expect((global as any).window.__lwcClearStylesheetCache).toBeTypeOf('function');
+        } finally {
+            // Restore original values
+            (global as any).process = originalProcess;
+            (global as any).window = originalWindow;
+        }
+    });
+});
diff --git a/packages/@lwc/ssr-client-utils/vitest.config.mjs b/packages/@lwc/ssr-client-utils/vitest.config.mjs
new file mode 100644
index 0000000000..d6edd9e05b
--- /dev/null
+++ b/packages/@lwc/ssr-client-utils/vitest.config.mjs
@@ -0,0 +1,11 @@
+import { defineConfig } from 'vitest/config';
+import sharedConfig from '../../../vitest.shared.mjs';
+
+export default defineConfig({
+    ...sharedConfig,
+    test: {
+        ...sharedConfig.test,
+        environment: 'jsdom',
+        setupFiles: [],
+    },
+});
diff --git a/vitest.workspace.mjs b/vitest.workspace.mjs
index 48faf843aa..0512350f46 100644
--- a/vitest.workspace.mjs
+++ b/vitest.workspace.mjs
@@ -12,6 +12,7 @@ export default defineWorkspace([
     'packages/@lwc/rollup-plugin',
     'packages/@lwc/shared',
     'packages/@lwc/signals',
+    'packages/@lwc/ssr-client-utils',
     'packages/@lwc/ssr-compiler',
     'packages/@lwc/ssr-runtime',
     'packages/@lwc/style-compiler',