feat: export defaultComponents for Streamdown components customization

apps/website/content/docs/components.mdx

@@ -59,30 +59,33 @@ You can override any of the following standard HTML components:
 Custom components receive all the props that the default components would receive, including:
 
 - `children` - The content to render
-- `className` - CSS class names from the Markdown AST (if applicable)
+- `className` - CSS class names from the Markdown AST (if applicable). This does not include Streamdown's built-in default styles.
 - `node` - The Markdown AST node (for advanced use cases)
 - Element-specific props (e.g., `href` for links, `src` for images)
 
 <Callout type="warn">
   Custom components **fully replace** the default implementations, including their built-in Tailwind styles. The `className` prop only contains classes from the Markdown AST (e.g., `language-js` on code elements) — it does **not** include the default styles that Streamdown normally applies.
 
-  If you need to preserve the default appearance, you must re-apply the styles yourself. See the [Styling](/docs/styling) documentation for the default classes, or use [CSS selectors with `data-streamdown` attributes](/docs/styling#global-css-targeting) instead of component overrides when you only need visual changes.
+  If you need to preserve the default appearance and behavior, import `defaultComponents` and render the built-in component inside your override. This keeps the default styles in sync with Streamdown updates and preserves attributes like `data-streamdown`.
 </Callout>
 
-For example, the default `h2` component applies `mt-6 mb-2 font-semibold text-2xl`. When you override it, those styles are lost unless you include them:
-
 ```tsx title="app/page.tsx"
+import { Streamdown, defaultComponents } from "streamdown";
+
+const DefaultH2 = defaultComponents.h2;
+
 <Streamdown
   components={{
     // ❌ Loses default spacing and font styles
     h2: ({ children }) => (
       <h2 className="text-blue-500">{children}</h2>
     ),
     // ✅ Preserves default styles alongside custom ones
-    h2: ({ children, className }) => (
-      <h2 className={`mt-6 mb-2 font-semibold text-2xl text-blue-500 ${className ?? ''}`}>
-        {children}
-      </h2>
+    h2: ({ className, ...restProps }) => (
+      <DefaultH2
+        {...restProps}
+        className={cn(className, 'text-blue-500')}
+      />
     ),
   }}
 >

packages/streamdown/index.tsx

@@ -63,6 +63,7 @@ export { CodeBlockHeader } from "./lib/code-block/header";
 export { CodeBlockSkeleton } from "./lib/code-block/skeleton";
 export { detectTextDirection } from "./lib/detect-direction";
 export type { IconMap } from "./lib/icon-context";
+export { defaultComponents };
 
 export type {
   AllowElement,

packages/streamdown/lib/components.tsx

@@ -1016,7 +1016,7 @@ const MemoParagraph = memo<ParagraphProps>(
 );
 MemoParagraph.displayName = "MarkdownParagraph";
 
-export const components: Options["components"] = {
+export const components = {
   ol: MemoOl,
   li: MemoLi,
   ul: MemoUl,
@@ -1048,4 +1048,4 @@ export const components: Options["components"] = {
   sub: MemoSub,
   p: MemoParagraph,
   section: MemoSection,
-};
+} satisfies Options["components"];