diff --git a/packages/toolbar/src/core/mount.tsx b/packages/toolbar/src/core/mount.tsx
index ac4b72c..dca92d9 100644
--- a/packages/toolbar/src/core/mount.tsx
+++ b/packages/toolbar/src/core/mount.tsx
@@ -61,24 +61,30 @@ function buildDom() {
   host.style.zIndex = '2147400100';
 
   const shadowRoot = host.attachShadow({ mode: 'open' });
+  if (!shadowRoot) {
+    throw new Error('[LaunchDarkly Toolbar] Failed to create shadow root');
+  }
+
   const reactMount = document.createElement('div');
 
   // Snapshot existing styles BEFORE the toolbar component loads
-  const existingStylesSnapshot = new Set(
-    Array.from(document.head.querySelectorAll('style')).map((el) => el.textContent || ''),
-  );
+  const existingStylesSnapshot = document.head
+    ? new Set(Array.from(document.head.querySelectorAll('style')).map((el) => el.textContent || ''))
+    : new Set();
 
   // Copy existing LaunchPad styles (including Gonfalon's) to shadow root
   // so toolbar has the base styles it needs
-  const existingStyles = Array.from(document.head.querySelectorAll('style'))
-    .filter((styleEl) => styleEl.textContent?.includes('--lp-') || styleEl.textContent?.includes('_'))
-    .map((styleEl) => styleEl.textContent || '')
-    .join('\n');
-
-  if (existingStyles) {
-    const style = document.createElement('style');
-    style.textContent = existingStyles;
-    shadowRoot.appendChild(style);
+  if (document.head) {
+    const existingStyles = Array.from(document.head.querySelectorAll('style'))
+      .filter((styleEl) => styleEl.textContent?.includes('--lp-') || styleEl.textContent?.includes('_'))
+      .map((styleEl) => styleEl.textContent || '')
+      .join('\n');
+
+    if (existingStyles) {
+      const style = document.createElement('style');
+      style.textContent = existingStyles;
+      shadowRoot.appendChild(style);
+    }
   }
 
   reactMount.dataset.name = 'react-mount';
@@ -108,8 +114,8 @@ function buildDom() {
             // We can remove immediately since we've already copied to shadow root
             try {
               styleEl.remove();
-            } catch {
-              // Ignore if already removed
+            } catch (error) {
+              console.warn('[LaunchDarkly Toolbar] Failed to remove style element from document.head:', error);
             }
           }
         }
@@ -117,7 +123,10 @@ function buildDom() {
     });
   });
 
-  observer.observe(document.head, { childList: true });
+  // Only observe document.head if it exists
+  if (document.head) {
+    observer.observe(document.head, { childList: true });
+  }
 
   // Stop observing after 500ms (toolbar should be fully loaded by then)
   setTimeout(() => observer.disconnect(), 500);
diff --git a/packages/toolbar/src/core/tests/mount.test.tsx b/packages/toolbar/src/core/tests/mount.test.tsx
new file mode 100644
index 0000000..6dbe2a3
--- /dev/null
+++ b/packages/toolbar/src/core/tests/mount.test.tsx
@@ -0,0 +1,262 @@
+import { expect, test, describe, vi, beforeEach, afterEach } from 'vitest';
+import mount from '../mount';
+import type { InitializationConfig } from '../../types';
+
+describe('mount', () => {
+  let rootNode: HTMLElement;
+  let mockConfig: InitializationConfig;
+
+  beforeEach(() => {
+    // Create a fresh root node for each test
+    rootNode = document.createElement('div');
+    document.body.appendChild(rootNode);
+
+    // Mock config
+    mockConfig = {
+      baseUrl: 'https://app.launchdarkly.com',
+      projectKey: 'test-project',
+      position: 'top-right',
+    };
+  });
+
+  afterEach(() => {
+    // Clean up DOM
+    document.body.innerHTML = '';
+    vi.clearAllMocks();
+  });
+
+  describe('Basic Mounting', () => {
+    test('creates toolbar host element with correct id', () => {
+      mount(rootNode, mockConfig);
+
+      const toolbarHost = document.getElementById('ld-toolbar');
+      expect(toolbarHost).not.toBeNull();
+      expect(toolbarHost?.tagName).toBe('DIV');
+    });
+
+    test('creates shadow root for style isolation', () => {
+      mount(rootNode, mockConfig);
+
+      const toolbarHost = document.getElementById('ld-toolbar');
+      expect(toolbarHost?.shadowRoot).not.toBeNull();
+      expect(toolbarHost?.shadowRoot?.mode).toBe('open');
+    });
+
+    test('creates react mount point inside shadow root', () => {
+      mount(rootNode, mockConfig);
+
+      const toolbarHost = document.getElementById('ld-toolbar');
+      const reactMount = toolbarHost?.shadowRoot?.getElementById('ld-toolbar-react-mount');
+
+      expect(reactMount).not.toBeNull();
+      expect(reactMount?.dataset.name).toBe('react-mount');
+    });
+
+    test('prevents multiple mounts with same DOM id', () => {
+      const cleanup1 = mount(rootNode, mockConfig);
+      const cleanup2 = mount(rootNode, mockConfig);
+
+      const toolbars = document.querySelectorAll('#ld-toolbar');
+      expect(toolbars.length).toBe(1);
+
+      cleanup1();
+      cleanup2();
+    });
+
+    test('applies correct styles to host element', () => {
+      mount(rootNode, mockConfig);
+
+      const toolbarHost = document.getElementById('ld-toolbar') as HTMLElement;
+      expect(toolbarHost.style.position).toBe('absolute');
+      expect(toolbarHost.style.zIndex).toBe('2147400100');
+      expect(toolbarHost.style.inset).toBe('0'); // Browsers normalize '0px' to '0'
+    });
+  });
+
+  describe('Style Isolation', () => {
+    test('copies existing LaunchPad styles to shadow root', () => {
+      // Add some LaunchPad styles to document.head
+      const styleEl = document.createElement('style');
+      styleEl.textContent = '.test { --lp-color-primary: blue; }';
+      document.head.appendChild(styleEl);
+
+      mount(rootNode, mockConfig);
+
+      const toolbarHost = document.getElementById('ld-toolbar');
+      const shadowStyles = toolbarHost?.shadowRoot?.querySelector('style');
+
+      expect(shadowStyles).not.toBeNull();
+      expect(shadowStyles?.textContent).toContain('--lp-');
+
+      styleEl.remove();
+    });
+
+    test('intercepts new toolbar styles and moves to shadow root', async () => {
+      const cleanup = mount(rootNode, mockConfig);
+
+      // Simulate toolbar injecting a style
+      const newStyle = document.createElement('style');
+      newStyle.textContent = '.button_abc123 { color: red; }';
+      document.head.appendChild(newStyle);
+
+      // Wait for MutationObserver to process
+      await new Promise((resolve) => setTimeout(resolve, 100));
+
+      const toolbarHost = document.getElementById('ld-toolbar');
+      const shadowStyles = Array.from(toolbarHost?.shadowRoot?.querySelectorAll('style') || []);
+      const hasButtonStyle = shadowStyles.some((style) => style.textContent?.includes('button_abc123'));
+
+      expect(hasButtonStyle).toBe(true);
+
+      // Verify style was removed from document.head
+      const headStyles = Array.from(document.head.querySelectorAll('style'));
+      const stillInHead = headStyles.some((style) => style.textContent?.includes('button_abc123'));
+      expect(stillInHead).toBe(false);
+
+      cleanup();
+    });
+
+    test('detects LaunchPad styles with --lp- prefix', async () => {
+      const cleanup = mount(rootNode, mockConfig);
+
+      const newStyle = document.createElement('style');
+      newStyle.textContent = ':root { --lp-spacing: 8px; }';
+      document.head.appendChild(newStyle);
+
+      // Wait for MutationObserver to process
+      await new Promise((resolve) => setTimeout(resolve, 100));
+
+      const toolbarHost = document.getElementById('ld-toolbar');
+      const shadowStyles = Array.from(toolbarHost?.shadowRoot?.querySelectorAll('style') || []);
+      const hasLpStyle = shadowStyles.some((style) => style.textContent?.includes('--lp-spacing'));
+
+      expect(hasLpStyle).toBe(true);
+
+      cleanup();
+    });
+
+    test('does not intercept host app styles without LaunchPad', async () => {
+      const cleanup = mount(rootNode, mockConfig);
+
+      const hostAppStyle = document.createElement('style');
+      hostAppStyle.textContent = '.my-app-class { color: blue; }';
+      document.head.appendChild(hostAppStyle);
+
+      // Wait for MutationObserver to process
+      await new Promise((resolve) => setTimeout(resolve, 100));
+
+      // Should still be in document.head
+      const headStyles = Array.from(document.head.querySelectorAll('style'));
+      const stillInHead = headStyles.some((style) => style.textContent?.includes('my-app-class'));
+      expect(stillInHead).toBe(true);
+
+      cleanup();
+      hostAppStyle.remove();
+    });
+
+    test('handles duplicate styles gracefully', async () => {
+      // Add existing style to head first
+      const existingStyle = document.createElement('style');
+      existingStyle.textContent = '.existing { --lp-color: blue; }';
+      document.head.appendChild(existingStyle);
+
+      const cleanup = mount(rootNode, mockConfig);
+
+      // Try adding the same style again (simulating toolbar re-injecting)
+      const duplicateStyle = document.createElement('style');
+      duplicateStyle.textContent = '.existing { --lp-color: blue; }';
+      document.head.appendChild(duplicateStyle);
+
+      await new Promise((resolve) => setTimeout(resolve, 100));
+
+      // Should handle gracefully without errors
+      const toolbarHost = document.getElementById('ld-toolbar');
+      const shadowStyles = Array.from(toolbarHost?.shadowRoot?.querySelectorAll('style') || []);
+
+      // Should have styles but no errors thrown
+      expect(shadowStyles.length).toBeGreaterThan(0);
+
+      existingStyle.remove();
+      cleanup();
+    });
+  });
+
+  describe('Cleanup', () => {
+    test('cleanup function removes toolbar from DOM', () => {
+      const cleanup = mount(rootNode, mockConfig);
+
+      expect(document.getElementById('ld-toolbar')).not.toBeNull();
+
+      cleanup();
+
+      expect(document.getElementById('ld-toolbar')).toBeNull();
+    });
+
+    test('cleanup function can be called multiple times safely', () => {
+      const cleanup = mount(rootNode, mockConfig);
+
+      cleanup();
+      expect(() => cleanup()).not.toThrow();
+    });
+  });
+
+  describe('Edge Cases and Error Handling', () => {
+    test('handles missing document.head gracefully without throwing', () => {
+      const originalHead = document.head;
+      let didThrow = false;
+
+      try {
+        // @ts-ignore - testing edge case
+        Object.defineProperty(document, 'head', {
+          get: () => null,
+          configurable: true,
+        });
+
+        mount(rootNode, mockConfig);
+      } catch {
+        didThrow = true;
+      } finally {
+        // Restore
+        Object.defineProperty(document, 'head', {
+          get: () => originalHead,
+          configurable: true,
+        });
+      }
+
+      // Should not throw even with null document.head
+      expect(didThrow).toBe(false);
+    });
+
+    test('handles style removal error gracefully', async () => {
+      const cleanup = mount(rootNode, mockConfig);
+
+      const newStyle = document.createElement('style');
+      newStyle.textContent = '.test_abc123 { color: red; }';
+      document.head.appendChild(newStyle);
+
+      // Spy on console.warn to verify error handling
+      const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+      // Make remove throw an error
+      const originalRemove = newStyle.remove;
+      newStyle.remove = vi.fn(() => {
+        throw new Error('Cannot remove');
+      });
+
+      // Wait for MutationObserver to process
+      await new Promise((resolve) => setTimeout(resolve, 100));
+
+      // Verify console.warn was called with error message
+      expect(warnSpy).toHaveBeenCalledWith(
+        expect.stringContaining('[LaunchDarkly Toolbar] Failed to remove style element'),
+        expect.any(Error),
+      );
+
+      // Restore and cleanup
+      warnSpy.mockRestore();
+      newStyle.remove = originalRemove;
+      newStyle.remove();
+      cleanup();
+    });
+  });
+});