diff --git a/frontend/src/components/markdown-renderer/markdown-to-react/utils/__snapshots__/node-to-react-transformer.spec.tsx.snap b/frontend/src/components/markdown-renderer/markdown-to-react/utils/__snapshots__/node-to-react-transformer.spec.tsx.snap
new file mode 100644
index 000000000..791185be2
--- /dev/null
+++ b/frontend/src/components/markdown-renderer/markdown-to-react/utils/__snapshots__/node-to-react-transformer.spec.tsx.snap
@@ -0,0 +1,13 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`node to react transformer replacement will prioritize a high priority replacer over a normal priority replacer 1`] = `
+<span>
+  Replacer X
+</span>
+`;
+
+exports[`node to react transformer replacement will prioritize a normal priority replacer over a low priority replacer 1`] = `
+<span>
+  Replacer Y
+</span>
+`;
diff --git a/frontend/src/extensions/essential-app-extensions/essential-app-extensions.ts b/frontend/src/extensions/essential-app-extensions/essential-app-extensions.ts
index f20663872..a75925c44 100644
--- a/frontend/src/extensions/essential-app-extensions/essential-app-extensions.ts
+++ b/frontend/src/extensions/essential-app-extensions/essential-app-extensions.ts
@@ -3,6 +3,7 @@
  *
  * SPDX-License-Identifier: AGPL-3.0-only
  */
+import { LegacySequenceDiagramAppExtension } from '../external-lib-app-extensions/sequence-diagram/legacy-sequence-diagram-app-extension'
 import { AlertAppExtension } from './alert/alert-app-extension'
 import { BasicMarkdownSyntaxAppExtension } from './basic-markdown-syntax/basic-markdown-syntax-app-extension'
 import { BlockquoteAppExtension } from './blockquote/blockquote-app-extension'
@@ -39,5 +40,6 @@ export const essentialAppExtensions = [
   new SpoilerAppExtension(),
   new TableOfContentsAppExtension(),
   new TaskListCheckboxAppExtension(),
-  new HeadlineAnchorsAppExtension()
+  new HeadlineAnchorsAppExtension(),
+  new LegacySequenceDiagramAppExtension()
 ]