fix(email-renderer): preserve section ancestry in parsed HTML

packages/email-renderer/test/email.test.ts

@@ -1,4 +1,5 @@
 import { describe, expect, test } from "bun:test";
+import { parse, type DefaultTreeAdapterMap } from "parse5";
 import {
   emailCoreBlocks,
   renderEmail,
@@ -55,6 +56,21 @@ const document: BlockDocument = {
 };
 
 describe("email renderer", () => {
+  test("browser HTML parsing preserves every block inside its section", async () => {
+    const rendered = await renderEmail(document);
+    expect(rendered.ok).toBe(true);
+    if (!rendered.ok) return;
+    const tree = parse(rendered.value.parts.html.content);
+    const ancestry = new Map<string, string[]>();
+    const walk = (node: DefaultTreeAdapterMap["node"], parents: string[]) => {
+      const id = "attrs" in node ? node.attrs.find(attr => attr.name === "data-block-id")?.value : undefined;
+      if (id) ancestry.set(id, parents);
+      if ("childNodes" in node) for (const child of node.childNodes) walk(child, id ? [...parents, id] : parents);
+    };
+    walk(tree, []);
+    expect(ancestry.get("heading")).toEqual(["root", "section"]);
+    expect(ancestry.get("button")).toEqual(["root", "section"]);
+  });
   test("renders deterministic HTML, text, AMP, and MJML", async () => {
     const first = await renderEmail(document, { amp: true, mjml: true });
     const second = await renderEmail(document, { amp: true, mjml: true });