Add Shadow DOM flattening and reorder js_code execution pipeline

- Add `flatten_shadow_dom` option to CrawlerRunConfig that serializes
  shadow DOM content into the light DOM before HTML capture. Uses a
  recursive serializer that resolves <slot> projections and strips
  only shadow-scoped <style> tags. Also injects an init script to
  force-open closed shadow roots via attachShadow patching.

- Move `js_code` execution to after `wait_for` + `delay_before_return_html`
  so user scripts run on the fully-hydrated page. Add `js_code_before_wait`
  for the less common case of triggering loading before waiting.

- Add JS snippet (flatten_shadow_dom.js), integration test, example,
  and documentation across all relevant doc files.

docs/md_v2/core/content-selection.md

@@ -183,6 +183,55 @@ if __name__ == "__main__":
 
 ---
 
+## 3.1 Flattening Shadow DOM
+
+Sites built with **Web Components** (Stencil, Lit, Shoelace, Angular Elements, etc.) render content inside [Shadow DOM](https://developer.mozilla.org/en-US/docs/Web/API/Web_components/Using_shadow_DOM) — an encapsulated sub-tree that is invisible to normal page serialization. The browser renders it on screen, but `page.content()` never includes it.
+
+Set `flatten_shadow_dom=True` to walk all shadow trees, resolve `<slot>` projections, and produce a single flat HTML document:
+
+```python
+config = CrawlerRunConfig(
+    # Flatten shadow DOM into the main document
+    flatten_shadow_dom=True,
+    # Give web components time to hydrate
+    wait_until="load",
+    delay_before_return_html=3.0,
+)
+```
+
+**Full example** — crawling a product page where specs live inside shadow roots:
+
+```python
+import asyncio
+from crawl4ai import AsyncWebCrawler, CrawlerRunConfig
+
+async def main():
+    config = CrawlerRunConfig(
+        flatten_shadow_dom=True,
+        wait_until="load",
+        delay_before_return_html=3.0,
+    )
+    async with AsyncWebCrawler() as crawler:
+        result = await crawler.arun(
+            url="https://store.boschrexroth.com/en/us/p/hydraulic-cylinder-r900999011",
+            config=config,
+        )
+        # Without flatten_shadow_dom: ~1 KB of markdown (breadcrumbs only)
+        # With flatten_shadow_dom:   ~33 KB (full product specs, downloads, etc.)
+        print(len(result.markdown.raw_markdown))
+
+if __name__ == "__main__":
+    asyncio.run(main())
+```
+
+When `flatten_shadow_dom=True` is set, Crawl4AI also injects an init script that force-opens **closed** shadow roots (by patching `Element.prototype.attachShadow`), so even components that use `mode: 'closed'` become accessible.
+
+> **Tip**: Web components need JavaScript to run before they render content (a process called *hydration*). Use `wait_until="load"` and a `delay_before_return_html` of 2–5 seconds to ensure components are fully hydrated before flattening.
+
+For a complete runnable example, see [`shadow_dom_crawling.py`](https://github.com/unclecode/crawl4ai/blob/main/docs/examples/shadow_dom_crawling.py).
+
+---
+
 ## 4. Structured Extraction Examples
 
 You can combine content selection with a more advanced extraction strategy. For instance, a **CSS-based** or **LLM-based** extraction strategy can run on the filtered HTML.