diff --git a/src/mount.ts b/src/mount.ts
new file mode 100644
index 0000000..4ebdc8a
--- /dev/null
+++ b/src/mount.ts
@@ -0,0 +1,90 @@
+import { h, render } from 'preact';
+import { Widget } from './ui/widget.js';
+import type { WidgetConfig } from './config.js';
+
+export type MountMode = 'classic' | 'shadow';
+
+export type MountHandle = {
+  root: HTMLElement;
+  shadowRoot?: ShadowRoot;
+};
+
+const ROOT_ID = 'messenzy-root';
+const SHADOW_INNER_ID = 'messenzy-shadow-inner';
+const SHADOW_STYLE_ID = 'messenzy-shadow-style';
+
+/**
+ * Mount the widget into the DOM.
+ *
+ * - classic mode: renders Preact directly into the host div in the light DOM.
+ *   The caller is responsible for ensuring CSS is in document.head.
+ * - shadow mode: attaches an open shadow root, injects CSS inside the shadow
+ *   tree (constructable stylesheets when supported, <style> fallback otherwise),
+ *   and renders Preact into a stable wrapper inside the shadow root.
+ *
+ * Both modes are idempotent: calling twice reuses the existing #messenzy-root
+ * (and its shadow root, in shadow mode) rather than recreating it.
+ */
+export function mountWidget(
+  config: WidgetConfig,
+  cssText: string,
+  mode: MountMode,
+): MountHandle {
+  const root = resolveOrCreateRoot();
+
+  if (mode !== 'shadow') {
+    render(h(Widget, { config }), root);
+    return { root };
+  }
+
+  const shadow = root.shadowRoot ?? root.attachShadow({ mode: 'open' });
+  applyStyles(shadow, cssText);
+
+  // Render into a stable inner wrapper so Preact's render() does not clobber
+  // the sibling <style> node when it diffs children.
+  let inner = shadow.getElementById(SHADOW_INNER_ID) as HTMLElement | null;
+  if (!inner) {
+    inner = document.createElement('div');
+    inner.id = SHADOW_INNER_ID;
+    shadow.appendChild(inner);
+  }
+  render(h(Widget, { config }), inner);
+  return { root, shadowRoot: shadow };
+}
+
+/**
+ * Inject CSS into a shadow root.
+ *
+ * Prefers constructable stylesheets (`adoptedStyleSheets`) — faster, no DOM
+ * mutation, sheet shareable across instances. Falls back to a deduped <style>
+ * child when constructable sheets are unavailable (jsdom, Safari < 16.4).
+ */
+export function applyStyles(shadow: ShadowRoot, cssText: string): void {
+  if (supportsConstructableStylesheets()) {
+    const sheet = new CSSStyleSheet();
+    sheet.replaceSync(cssText);
+    shadow.adoptedStyleSheets = [sheet];
+    return;
+  }
+  if (shadow.getElementById(SHADOW_STYLE_ID)) return;
+  const style = document.createElement('style');
+  style.id = SHADOW_STYLE_ID;
+  style.textContent = cssText;
+  shadow.prepend(style);
+}
+
+function resolveOrCreateRoot(): HTMLElement {
+  const existing = document.getElementById(ROOT_ID);
+  if (existing) return existing;
+  const root = document.createElement('div');
+  root.id = ROOT_ID;
+  document.body.appendChild(root);
+  return root;
+}
+
+function supportsConstructableStylesheets(): boolean {
+  return (
+    typeof CSSStyleSheet !== 'undefined' &&
+    'replaceSync' in CSSStyleSheet.prototype
+  );
+}
diff --git a/tests/unit/mount.test.ts b/tests/unit/mount.test.ts
new file mode 100644
index 0000000..c2443b8
--- /dev/null
+++ b/tests/unit/mount.test.ts
@@ -0,0 +1,120 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+
+// Stub the Widget component so mountWidget does not boot the WebSocket
+// transport (jsdom has no WebSocket implementation). We only verify mount
+// mechanics here — Widget rendering is a separate concern.
+vi.mock('@/ui/widget', () => ({
+  Widget: () => null,
+}));
+
+import { mountWidget, applyStyles } from '@/mount';
+import type { WidgetConfig } from '@/config';
+
+const baseConfig: WidgetConfig = {
+  botId: 'test-bot',
+  apiKey: 'test-key',
+  serverUrl: 'https://test.local',
+  position: 'bottom-right',
+  shadow: false,
+};
+
+const TEST_CSS = '.messenzy-widget { color: tomato; }';
+
+beforeEach(() => {
+  document.head.innerHTML = '';
+  document.body.innerHTML = '';
+});
+
+describe('mountWidget — classic mode', () => {
+  it('creates #messenzy-root in document.body when absent', () => {
+    mountWidget(baseConfig, TEST_CSS, 'classic');
+    const root = document.getElementById('messenzy-root');
+    expect(root).not.toBeNull();
+    expect(root?.parentElement).toBe(document.body);
+  });
+
+  it('reuses an existing #messenzy-root', () => {
+    const existing = document.createElement('div');
+    existing.id = 'messenzy-root';
+    document.body.appendChild(existing);
+    const handle = mountWidget(baseConfig, TEST_CSS, 'classic');
+    expect(handle.root).toBe(existing);
+  });
+
+  it('returns a handle without shadowRoot', () => {
+    const handle = mountWidget(baseConfig, TEST_CSS, 'classic');
+    expect(handle.shadowRoot).toBeUndefined();
+  });
+
+  it('does not attach a shadow root to the host', () => {
+    mountWidget(baseConfig, TEST_CSS, 'classic');
+    const root = document.getElementById('messenzy-root');
+    expect(root?.shadowRoot).toBeNull();
+  });
+});
+
+describe('mountWidget — shadow mode', () => {
+  it('creates the host and attaches an open shadow root', () => {
+    const handle = mountWidget(baseConfig, TEST_CSS, 'shadow');
+    const root = document.getElementById('messenzy-root');
+    expect(root).not.toBeNull();
+    expect(root?.shadowRoot).not.toBeNull();
+    expect(handle.shadowRoot).toBe(root?.shadowRoot);
+  });
+
+  it('reuses an already-attached shadow root (HMR/idempotency)', () => {
+    const existing = document.createElement('div');
+    existing.id = 'messenzy-root';
+    const preAttached = existing.attachShadow({ mode: 'open' });
+    document.body.appendChild(existing);
+    const handle = mountWidget(baseConfig, TEST_CSS, 'shadow');
+    expect(handle.shadowRoot).toBe(preAttached);
+  });
+
+  it('injects styles into the shadow root, not document.head', () => {
+    mountWidget(baseConfig, TEST_CSS, 'shadow');
+    expect(document.getElementById('messenzy-styles')).toBeNull();
+    const shadow = document.getElementById('messenzy-root')?.shadowRoot;
+    const styleNode = shadow?.querySelector('style');
+    expect(styleNode).not.toBeNull();
+    expect(styleNode?.textContent).toContain('tomato');
+  });
+
+  it('keeps the <style> intact when Preact renders (uses inner wrapper)', () => {
+    mountWidget(baseConfig, TEST_CSS, 'shadow');
+    const shadow = document.getElementById('messenzy-root')?.shadowRoot;
+    // The shadow root must contain both the style node AND a separate
+    // wrapper for the Preact render target — otherwise render() would
+    // clobber the <style>.
+    expect(shadow?.querySelector('style')).not.toBeNull();
+    expect(shadow?.children.length).toBeGreaterThanOrEqual(2);
+  });
+});
+
+describe('applyStyles', () => {
+  let shadow: ShadowRoot;
+
+  beforeEach(() => {
+    const host = document.createElement('div');
+    document.body.appendChild(host);
+    shadow = host.attachShadow({ mode: 'open' });
+  });
+
+  it('appends a <style> node into the shadow root (jsdom fallback path)', () => {
+    applyStyles(shadow, TEST_CSS);
+    const style = shadow.querySelector('style');
+    expect(style).not.toBeNull();
+    expect(style?.textContent).toBe(TEST_CSS);
+  });
+
+  it('is idempotent — repeated calls keep one <style>', () => {
+    applyStyles(shadow, TEST_CSS);
+    applyStyles(shadow, TEST_CSS);
+    expect(shadow.querySelectorAll('style')).toHaveLength(1);
+  });
+
+  // The constructable-stylesheets path (`adoptedStyleSheets`) is exercised
+  // in real browsers with full CSSOM support. jsdom does not implement it,
+  // so the unit suite verifies only the <style>-fallback branch. Manual
+  // smoke-test in a real browser confirms the modern path.
+});