diff --git a/tools/web-ux/browser-tests/memory_document_fixture_server.ts b/tools/web-ux/browser-tests/memory_document_fixture_server.ts
index 3d9c44183..ba1c24614 100644
--- a/tools/web-ux/browser-tests/memory_document_fixture_server.ts
+++ b/tools/web-ux/browser-tests/memory_document_fixture_server.ts
@@ -25,7 +25,7 @@ const paragraphs = Array.from(
 const bodyMd =
   `# Workspace Memory\n\nThis document is read-only.\n\n| Very wide heading | Another heading | Decision |\n| --- | --- | --- |\n| ${
     "wide-value-".repeat(16)
-  } | stable | keep scrolling local |\n\n\`\`\`rust\nfn example() { println!("${
+  } | stable | keep scrolling local |\n\n##### Deep document heading\n\n###### Deepest document heading\n\n\`\`\`rust\nfn example() { println!("${
     "wide-code-".repeat(18)
   }"); }\n\`\`\`\n\n[Safe link](https://example.com) [Unsafe link](javascript:alert(1))\n\n<img src=x onerror=alert(1)>\n\n${paragraphs}\n\n## T-661 UNIQUE END MARKER`;
 
diff --git a/tools/web-ux/browser-tests/memory_document_shell_test.ts b/tools/web-ux/browser-tests/memory_document_shell_test.ts
index 75f13de3d..54c8d403f 100644
--- a/tools/web-ux/browser-tests/memory_document_shell_test.ts
+++ b/tools/web-ux/browser-tests/memory_document_shell_test.ts
@@ -165,6 +165,21 @@ Deno.test("production Memory shell renders safe Markdown and scrolls to its end
           );
           assertEquals(await page.locator("blockquote").count() > 0, true);
           assertEquals(await page.locator("ul li").count() > 0, true);
+          for (
+            const heading of [
+              { selector: "h5", text: "Deep document heading" },
+              { selector: "h6", text: "Deepest document heading" },
+            ]
+          ) {
+            const element = page.locator(heading.selector, { hasText: heading.text });
+            assertEquals(await element.count(), 1);
+            assert(
+              await element.evaluate((node) =>
+                Number.parseFloat((globalThis as any).getComputedStyle(node).fontSize) >= 12
+              ),
+              `${heading.selector} fell below the design language minimum text size`,
+            );
+          }
           assertEquals(await page.locator('a[href="https://example.com"]').count(), 1);
           assertEquals(await page.locator('a[href^="javascript:"]').count(), 0);
           assertEquals(await page.locator("article img, article script").count(), 0);
diff --git a/web/workspace/src/lib/workspace/markdown/DocumentMarkdown.browser.spec.ts b/web/workspace/src/lib/workspace/markdown/DocumentMarkdown.browser.spec.ts
index 16b1c5084..0c1fe7808 100644
--- a/web/workspace/src/lib/workspace/markdown/DocumentMarkdown.browser.spec.ts
+++ b/web/workspace/src/lib/workspace/markdown/DocumentMarkdown.browser.spec.ts
@@ -10,6 +10,10 @@ test("renders document Markdown semantics through the shared safe policy", async
   const source = [
     "# Memory heading",
     "",
+    "##### Deep heading",
+    "",
+    "###### Deepest heading",
+    "",
     "A paragraph with **strong context** and `inline-code`.",
     "",
     "- first item",
@@ -36,10 +40,15 @@ test("renders document Markdown semantics through the shared safe policy", async
     expect(container.querySelector("table")).not.toBeNull();
   });
 
+  expect(container.querySelector("h5")?.textContent).toBe("Deep heading");
+  expect(container.querySelector("h6")?.textContent).toBe("Deepest heading");
   expect(container.querySelector("strong")?.textContent).toBe("strong context");
   expect(container.querySelectorAll("li")).toHaveLength(2);
-  expect(container.querySelector("blockquote")?.textContent).toContain("durable quote");
-  expect(container.querySelector('a[href="https://example.com"]')).not.toBeNull();
+  expect(container.querySelector("blockquote")?.textContent).toContain(
+    "durable quote",
+  );
+  expect(container.querySelector('a[href="https://example.com"]')).not
+    .toBeNull();
   expect(container.querySelectorAll("a")).toHaveLength(1);
   expect(container.querySelector("img")).toBeNull();
   expect(container.querySelector("script")).toBeNull();
@@ -52,13 +61,16 @@ test("renders document Markdown semantics through the shared safe policy", async
   );
   expect(tableRegion?.tabIndex).toBe(0);
   expect(codeRegion?.tabIndex).toBe(0);
-  expect(codeRegion?.querySelector("pre > code")?.textContent).toContain("println!");
+  expect(codeRegion?.querySelector("pre > code")?.textContent).toContain(
+    "println!",
+  );
 });
 
 test("wraps long prose and identifiers without turning them into document-level controls", async () => {
   const longIdentifier = "memory-identifier-".repeat(40);
   const { container } = render(DocumentMarkdown, {
-    text: `Long value: ${longIdentifier}\n\n<details open><summary>unsafe HTML</summary></details>`,
+    text:
+      `Long value: ${longIdentifier}\n\n<details open><summary>unsafe HTML</summary></details>`,
   });
 
   await waitFor(() => expect(container.textContent).toContain(longIdentifier));
diff --git a/web/workspace/src/lib/workspace/markdown/DocumentMarkdown.svelte b/web/workspace/src/lib/workspace/markdown/DocumentMarkdown.svelte
index 6eeef04c2..77add1dce 100644
--- a/web/workspace/src/lib/workspace/markdown/DocumentMarkdown.svelte
+++ b/web/workspace/src/lib/workspace/markdown/DocumentMarkdown.svelte
@@ -41,7 +41,9 @@
 
   :global(.document-markdown h2),
   :global(.document-markdown h3),
-  :global(.document-markdown h4) {
+  :global(.document-markdown h4),
+  :global(.document-markdown h5),
+  :global(.document-markdown h6) {
     margin: var(--space-6) 0 var(--space-2);
     color: var(--text-strong);
     font-size: var(--font-size-body);
@@ -53,7 +55,9 @@
   }
 
   :global(.document-markdown h3),
-  :global(.document-markdown h4) {
+  :global(.document-markdown h4),
+  :global(.document-markdown h5),
+  :global(.document-markdown h6) {
     font-weight: 600;
   }