feat(markdown): support registering custom containers

New containers can be registered via
`markdown.container.customContainers`, mapping a container name to its
default title. Registered names work as `::: name` blocks and as
GitHub-style alerts (`> [!NAME]`), with custom titles, attributes, and
no-title behaving like the built-in types. Styling is left to the theme
via `.custom-block.name`.

close #3591
close #3603
close #4228

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

__tests__/unit/node/markdown/plugins/containers.test.ts

@@ -126,6 +126,39 @@ describe('node/markdown/plugins/containers', () => {
     `)
   })
 
+  test('registers custom containers from container options', async () => {
+    const src = [
+      '::: success',
+      'You have completed the walkthrough!',
+      ':::',
+      '',
+      '::: success Well done {no-title}',
+      'content',
+      ':::'
+    ].join('\n')
+    expect(
+      await render(src, {
+        container: { customContainers: { success: 'SUCCESS' } }
+      })
+    ).toMatchInlineSnapshot(`
+      "<div  class="success custom-block"><p class="custom-block-title custom-block-title-default">SUCCESS</p>
+      <p>You have completed the walkthrough!</p>
+      </div>
+      <div  class="success custom-block">
+      <p>content</p>
+      </div>
+      "
+    `)
+  })
+
+  test('rejects invalid custom container names', async () => {
+    for (const name of ['raw', 'v-pre', 'code-group', 'Bad Name', 'UPPER']) {
+      await expect(
+        render('text', { container: { customContainers: { [name]: 'X' } } })
+      ).rejects.toThrow('Invalid custom container name')
+    }
+  })
+
   test('supports attrs on the fence line', async () => {
     const src = [
       '::: details Click me {open}',
@@ -362,6 +395,22 @@ describe('node/markdown/plugins/containers (github alerts)', () => {
     `)
   })
 
+  test('renders custom containers as alerts', async () => {
+    expect(
+      await render('> [!SUCCESS]\n> done\n\n> [!success] With title\n> done', {
+        container: { customContainers: { success: 'SUCCESS' } }
+      })
+    ).toMatchInlineSnapshot(`
+      "<div class="success custom-block github-alert"><p class="custom-block-title">SUCCESS</p>
+      <p>done</p>
+      </div>
+      <div class="success custom-block github-alert"><p class="custom-block-title">With title</p>
+      <p>done</p>
+      </div>
+      "
+    `)
+  })
+
   test('supports block content and lazy continuation', async () => {
     const src = [
       '> [!NOTE]',

docs/en/guide/markdown.md

@@ -243,6 +243,47 @@ export default defineConfig({
 })
 ```
 
+### Registering New Containers
+
+Beyond the built-in types, you can register additional containers by mapping their names to their default titles:
+
+```ts
+// config.ts
+export default defineConfig({
+  // ...
+  markdown: {
+    container: {
+      customContainers: {
+        success: 'SUCCESS'
+      }
+    }
+  }
+  // ...
+})
+```
+
+Registered names work like the built-in ones - including custom titles, attributes, and the [GitHub-style alert syntax](#github-flavored-alerts):
+
+```md
+::: success
+You have completed the walkthrough!
+:::
+
+> [!SUCCESS] Custom title
+> This renders the same way.
+```
+
+New containers ship without any styling, so add some in your theme using the container name as the class. For this example, the default theme's palette already provides fitting colors:
+
+```css
+/* .vitepress/theme/custom.css */
+.custom-block.success {
+  border-color: transparent;
+  color: var(--vp-c-text-1);
+  background-color: var(--vp-c-success-soft);
+}
+```
+
 ### Nesting
 
 The `:::` markers follow the same rules as fenced code blocks (` ``` `): a fence is only closed by a matching fence that is **at least as long** as the one that opened it. To nest containers (or to mix them with [code groups](#code-groups)) make the outer fence longer than the ones inside it.
@@ -351,7 +392,7 @@ Wraps in a `<div class="vp-raw">`
 
 ## GitHub-flavored Alerts
 
-VitePress also supports [GitHub-flavored alerts](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts) to render as callouts. They will be rendered the same as the [custom containers](#custom-containers).
+VitePress also supports [GitHub-flavored alerts](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts) to render as callouts. They will be rendered the same as the [custom containers](#custom-containers). Unlike on GitHub, text placed right after the marker becomes the title of the alert (`> [!NOTE] Custom Title`), and [containers you registered yourself](#registering-new-containers) work here too.
 
 ```md
 > [!NOTE]

src/node/markdown/markdown.ts

@@ -251,8 +251,9 @@ export interface MarkdownOptions extends MarkdownItAsyncOptions {
    */
   math?: boolean | any
   /**
-   * Custom labels for the built-in containers (`::: tip` etc.). Also used
-   * as the default titles of GitHub-flavored alerts.
+   * Custom labels for the built-in containers (`::: tip` etc.) and
+   * additional user-defined containers. Labels are also used as the
+   * default titles of GitHub-flavored alerts.
    * @see https://vitepress.dev/guide/markdown#custom-containers
    */
   container?: ContainerOptions

src/node/markdown/plugins/containers.ts

@@ -14,6 +14,14 @@ export interface ContainerOptions {
   detailsLabel?: string
   importantLabel?: string
   cautionLabel?: string
+  /**
+   * Additional containers to register, mapping the container name to its
+   * default title. Registered names work both as `::: name` blocks and as
+   * GitHub-style alerts (`> [!NAME]`), and are styleable in the theme via
+   * `.custom-block.name`. Names must be lowercase and may only contain
+   * letters, numbers, hyphens, and underscores.
+   */
+  customContainers?: Record<string, string>
 }
 
 export const containerPlugin = (
@@ -49,7 +57,7 @@ export const containerPlugin = (
 }
 
 function resolveTitles(options?: ContainerOptions): Record<string, string> {
-  return {
+  const titles: Record<string, string> = {
     tip: options?.tipLabel || 'TIP',
     info: options?.infoLabel || 'INFO',
     warning: options?.warningLabel || 'WARNING',
@@ -59,6 +67,18 @@ function resolveTitles(options?: ContainerOptions): Record<string, string> {
     important: options?.importantLabel || 'IMPORTANT',
     caution: options?.cautionLabel || 'CAUTION'
   }
+  for (const [name, title] of Object.entries(options?.customContainers ?? {})) {
+    if (
+      !/^[a-z0-9_-]+$/.test(name) ||
+      ['v-pre', 'raw', 'code-group'].includes(name)
+    )
+      throw new Error(
+        `Invalid custom container name: "${name}". Names must be lowercase ` +
+          `([a-z0-9_-]) and cannot be "v-pre", "raw", or "code-group".`
+      )
+    titles[name] = title
+  }
+  return titles
 }
 
 function createOpenRender(