diff --git a/src/trackChangesModel.ts b/src/trackChangesModel.ts
index eb31cf2..17f6d54 100644
--- a/src/trackChangesModel.ts
+++ b/src/trackChangesModel.ts
@@ -121,3 +121,71 @@ export function diffBlocks(baselineText: string, currentText: string): BlockOp[]
   }
   return ops;
 }
+
+const md = new MarkdownIt({ html: true, linkify: false, breaks: false });
+// mermaid fences → <pre class="mermaid">SRC</pre> for client-side rendering; all
+// other fences fall through to markdown-it's default (escaped <pre><code>).
+const defaultFence = md.renderer.rules.fence!.bind(md.renderer.rules);
+md.renderer.rules.fence = (tokens, idx, options, env, self) => {
+  const info = tokens[idx].info.trim().split(/\s+/)[0].toLowerCase();
+  if (info === "mermaid") {
+    return `<pre class="mermaid">${md.utils.escapeHtml(tokens[idx].content.replace(/\n$/, ""))}</pre>\n`;
+  }
+  return defaultFence(tokens, idx, options, env, self);
+};
+
+/** Build a markdown string with inline <ins>/<del> from a word-level prose diff. */
+function wordMergedMarkdown(beforeRaw: string, afterRaw: string): string {
+  return diffWords(beforeRaw, afterRaw)
+    .map((part) => {
+      if (part.added) return `<ins>${part.value}</ins>`;
+      if (part.removed) return `<del>${part.value}</del>`;
+      return part.value;
+    })
+    .join("");
+}
+
+function safeRender(src: string): string {
+  try {
+    return md.render(src);
+  } catch (err) {
+    const message = err instanceof Error ? err.message : String(err);
+    return `<div class="cw-error">Could not render this block: ${md.utils.escapeHtml(message)}</div>`;
+  }
+}
+
+function renderOp(op: BlockOp): string {
+  let cls: string;
+  let inner: string;
+  let badge = "";
+  switch (op.kind) {
+    case "unchanged":
+      cls = "cw-unchanged";
+      inner = safeRender(op.block.raw);
+      break;
+    case "added":
+      cls = "cw-added";
+      inner = safeRender(op.block.raw);
+      if (op.block.type !== "prose") badge = '<span class="cw-badge">added</span>';
+      break;
+    case "removed":
+      cls = "cw-removed";
+      inner = safeRender(op.block.raw);
+      break;
+    case "changed":
+      cls = "cw-changed";
+      if (op.atomic) {
+        inner = safeRender(op.block.raw); // the NEW block, whole (INV-23)
+        badge = '<span class="cw-badge">changed</span>';
+      } else {
+        inner = safeRender(wordMergedMarkdown(op.before.raw, op.block.raw));
+      }
+      break;
+  }
+  return `<div class="cw-blk ${cls}">${badge}${inner}</div>`;
+}
+
+/** Pure entry point: annotated HTML body for the preview (INV-22). */
+export function renderTrackChanges(baselineText: string, currentText: string): string {
+  return diffBlocks(baselineText, currentText).map(renderOp).join("\n");
+}
diff --git a/test/trackChangesModel.test.ts b/test/trackChangesModel.test.ts
index c969fea..b07bc04 100644
--- a/test/trackChangesModel.test.ts
+++ b/test/trackChangesModel.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { splitBlocks, diffBlocks } from "../src/trackChangesModel";
+import { splitBlocks, diffBlocks, renderTrackChanges } from "../src/trackChangesModel";
 
 describe("splitBlocks", () => {
   it("splits prose paragraphs on blank lines, dropping empties", () => {
@@ -76,3 +76,49 @@ describe("diffBlocks", () => {
     expect(k).toContain("unchanged");
   });
 });
+
+describe("renderTrackChanges", () => {
+  it("wraps each block in a cw-blk div with its kind class", () => {
+    const html = renderTrackChanges("Alpha.\n", "Alpha.\n\nBravo.\n");
+    expect(html).toContain('class="cw-blk cw-unchanged"');
+    expect(html).toContain('class="cw-blk cw-added"');
+  });
+
+  it("emits inline <ins>/<del> for a prose modification", () => {
+    const html = renderTrackChanges("The quick fox.\n", "The slow fox.\n");
+    expect(html).toContain("<ins>");
+    expect(html).toContain("<del>");
+    expect(html).toContain("cw-changed");
+  });
+
+  it("renders a changed CODE fence atomically: cw-changed, no inline ins/del", () => {
+    const html = renderTrackChanges("```ts\nconst a = 1;\n```\n", "```ts\nconst a = 2;\n```\n");
+    expect(html).toContain("cw-changed");
+    expect(html).not.toContain("<ins>");
+    expect(html).not.toContain("<del>");
+    expect(html).toContain("cw-badge");
+  });
+
+  it('emits <pre class="mermaid"> for a mermaid fence + a changed badge', () => {
+    const html = renderTrackChanges(
+      "```mermaid\nflowchart LR\n a-->b\n```\n",
+      "```mermaid\nflowchart LR\n a-->c\n```\n",
+    );
+    expect(html).toContain('<pre class="mermaid">');
+    expect(html).toContain("a--&gt;c"); // current source, escaped
+    expect(html).toContain("cw-badge");
+  });
+
+  it("unchanged doc renders with no marks", () => {
+    const html = renderTrackChanges("Alpha.\n", "Alpha.\n");
+    expect(html).not.toContain("cw-added");
+    expect(html).not.toContain("cw-removed");
+    expect(html).not.toContain("cw-changed");
+  });
+
+  it("is deterministic (same inputs → identical HTML) (INV-22)", () => {
+    const a = renderTrackChanges("X.\n\nY.\n", "X.\n\nZ.\n");
+    const b = renderTrackChanges("X.\n\nY.\n", "X.\n\nZ.\n");
+    expect(a).toBe(b);
+  });
+});