diff --git a/src/editor/lsp/markdown.test.ts b/src/editor/lsp/markdown.test.ts
index d73575a6b..c3b5e2f51 100644
--- a/src/editor/lsp/markdown.test.ts
+++ b/src/editor/lsp/markdown.test.ts
@@ -7,6 +7,39 @@ async function freshMarkdown() {
 }
 
 describe("markdown rendering", () => {
+  it.each([
+    "<style>body{outline:7px solid red}</style>",
+    '<form action="https://example.com/x"><button>go</button></form>',
+    '<div style="position:fixed">x</div>',
+    '<link rel="stylesheet" href="https://example.com/x.css">',
+    '<meta http-equiv="refresh" content="0;url=https://example.com">',
+    '<base href="https://example.com/">',
+    '<iframe src="https://example.com"></iframe>',
+    '<button formaction="https://example.com/x">go</button>',
+  ])("keeps raw markup from reaching the page: %s", async (raw) => {
+    const md = await freshMarkdown();
+    await md.loadMarkdown();
+    const root = document.createElement("div");
+    root.innerHTML = md.markupToHtml({ kind: "markdown", value: `text\n\n${raw}\n` });
+    expect(root.querySelector("style, form, link, meta, base, iframe, [style], [formaction]")).toBeNull();
+    expect(root.textContent).not.toContain("outline");
+  });
+
+  it("keeps the links and tables comments and docs rely on", async () => {
+    const md = await freshMarkdown();
+    await md.loadMarkdown();
+    const root = document.createElement("div");
+    root.innerHTML = md.markupToHtml({
+      kind: "markdown",
+      value: "[doc](jdt://contents/a.class) [web](https://example.com)\n\n| A |\n| - |\n| 1 |\n",
+    });
+    expect([...root.querySelectorAll("a")].map((a) => a.getAttribute("href"))).toEqual([
+      "jdt://contents/a.class",
+      "https://example.com",
+    ]);
+    expect(root.querySelector("td")?.textContent).toBe("1");
+  });
+
   it("shows plain text until the parser loads, then the rendered markup", async () => {
     const md = await freshMarkdown();
     const el = md.renderMarkup({ kind: "markdown", value: "**bold** <img src=x onerror=alert(1)>" });
diff --git a/src/editor/lsp/markdownParser.ts b/src/editor/lsp/markdownParser.ts
index 2f4698aaf..e4473eda6 100644
--- a/src/editor/lsp/markdownParser.ts
+++ b/src/editor/lsp/markdownParser.ts
@@ -9,5 +9,11 @@ const marked = new Marked({ gfm: true, breaks: false });
 
 export function markdownToSafeHtml(markdown: string): string {
   const html = marked.parse(markdown, { async: false }) as string;
-  return DOMPurify.sanitize(html, { USE_PROFILES: { html: true }, ALLOWED_URI_REGEXP: ALLOWED_URI });
+  // Comments and docs come from anyone; a `<style>` here would restyle the whole IDE.
+  return DOMPurify.sanitize(html, {
+    USE_PROFILES: { html: true },
+    ALLOWED_URI_REGEXP: ALLOWED_URI,
+    FORBID_TAGS: ["style", "form"],
+    FORBID_ATTR: ["style"],
+  });
 }
diff --git a/src/features/agents/Markdown.test.tsx b/src/features/agents/Markdown.test.tsx
index faed31748..4d98e26cf 100644
--- a/src/features/agents/Markdown.test.tsx
+++ b/src/features/agents/Markdown.test.tsx
@@ -11,6 +11,37 @@ vi.mock("@/platform/openUrl", () => ({ openExternalUrl }));
 
 beforeEach(() => vi.clearAllMocks());
 
+it.each([
+  "<style>body{outline:7px solid red}</style>",
+  '<form action="https://example.com/x"><button>go</button></form>',
+  '<div style="position:fixed">x</div>',
+  '<link rel="stylesheet" href="https://example.com/x.css">',
+  '<meta http-equiv="refresh" content="0;url=https://example.com">',
+  '<base href="https://example.com/">',
+  '<iframe src="https://example.com"></iframe>',
+  '<button formaction="https://example.com/x">go</button>',
+])("keeps raw markup from reaching the page: %s", (raw) => {
+  const { container } = render(<Markdown text={`text\n\n${raw}`} cwd="/repo" />);
+  expect(container.querySelector("style, form, link, meta, base, iframe, [style], [formaction]")).toBeNull();
+  expect(container.textContent).not.toContain("outline");
+});
+
+it("ignores data attributes written in a reply, keeping its own", () => {
+  const { container } = render(
+    <Markdown
+      text={'<button data-copy="evil">x</button>\n\n<code data-path="/etc/passwd">y</code>\n\n```ts\nconst a = 1;\n```'}
+      cwd="/repo"
+    />,
+  );
+  expect(container.querySelector("code[data-path]")).toBeNull();
+  fireEvent.click(screen.getByRole("button", { name: "x" }));
+  fireEvent.click(container.querySelector("code")!);
+  expect(openFile).not.toHaveBeenCalled();
+  expect(
+    [...container.querySelectorAll<HTMLElement>("button[data-copy]")].map((button) => button.dataset.copy),
+  ).toEqual(["const a = 1;"]);
+});
+
 it.each([
   ["/Users/thomas/vmos-shots/tiktok-home.jpg", "/Users/thomas/vmos-shots/tiktok-home.jpg", undefined],
   ["images/home.jpg", "/repo/images/home.jpg", undefined],
diff --git a/src/features/agents/Markdown.tsx b/src/features/agents/Markdown.tsx
index fe80c6828..a0da3f508 100644
--- a/src/features/agents/Markdown.tsx
+++ b/src/features/agents/Markdown.tsx
@@ -19,7 +19,15 @@ const marked = new Marked({ gfm: true, breaks: true });
 
 export function markdownHtml(text: string): string {
   const html = marked.parse(text, { async: false }) as string;
-  const root = DOMPurify.sanitize(html, { USE_PROFILES: { html: true }, RETURN_DOM: true }) as HTMLElement;
+  // Replies can quote repository content. Styles would reach the whole IDE, and the click
+  // handlers below trust `data-copy` / `data-path`, which only `enhance` may set.
+  const root = DOMPurify.sanitize(html, {
+    USE_PROFILES: { html: true },
+    RETURN_DOM: true,
+    FORBID_TAGS: ["style", "form"],
+    FORBID_ATTR: ["style"],
+    ALLOW_DATA_ATTR: false,
+  }) as HTMLElement;
   root.querySelectorAll("li").forEach(wrapLooseText);
   return root.innerHTML;
 }
diff --git a/src/features/notebook/security.test.ts b/src/features/notebook/security.test.ts
index 6ef2b8298..e8282a3ec 100644
--- a/src/features/notebook/security.test.ts
+++ b/src/features/notebook/security.test.ts
@@ -4,6 +4,21 @@ import { markdownRoot, resolveMarkdownAttachments, sanitizedSvgUrl, staticOutput
 const parse = (html: string) => new DOMParser().parseFromString(html, "text/html");
 
 describe("static notebook output sanitization", () => {
+  it.each([
+    "<style>body{outline:7px solid red}</style>",
+    '<form action="https://example.com/x"><button>go</button></form>',
+    '<div style="position:fixed">x</div>',
+    '<link rel="stylesheet" href="https://example.com/x.css">',
+    '<meta http-equiv="refresh" content="0;url=https://example.com">',
+    '<base href="https://example.com/">',
+    '<iframe src="https://example.com"></iframe>',
+    '<button formaction="https://example.com/x">go</button>',
+  ])("keeps raw Markdown markup from reaching the page: %s", (raw) => {
+    const root = markdownRoot(`text\n\n${raw}\n`);
+    expect(root.querySelector("style, form, link, meta, base, iframe, [style], [formaction]")).toBeNull();
+    expect(root.textContent).not.toContain("outline");
+  });
+
   it("retains table content while removing executable, navigation and external-resource surfaces", () => {
     const document = parse(
       staticOutputDocument(`
diff --git a/src/features/viewers/markdown/render.test.ts b/src/features/viewers/markdown/render.test.ts
index 28031903f..6519c8bc5 100644
--- a/src/features/viewers/markdown/render.test.ts
+++ b/src/features/viewers/markdown/render.test.ts
@@ -30,6 +30,32 @@ describe("markdown preview rendering", () => {
     expect(root.querySelector("a")?.getAttribute("href")).toBeNull();
   });
 
+  it.each([
+    "<style>body{outline:7px solid red}</style>",
+    '<form action="https://example.com/x"><button>go</button></form>',
+    '<div style="position:fixed">x</div>',
+    '<link rel="stylesheet" href="https://example.com/x.css">',
+    '<meta http-equiv="refresh" content="0;url=https://example.com">',
+    '<base href="https://example.com/">',
+    '<iframe src="https://example.com"></iframe>',
+    '<button formaction="https://example.com/x">go</button>',
+  ])("keeps raw markup from reaching the page: %s", (raw) => {
+    const root = document.createElement("div");
+    root.innerHTML = renderMarkdown(`text\n\n${raw}\n`);
+    expect(root.querySelector("style, form, link, meta, base, iframe, [style], [formaction]")).toBeNull();
+    expect(root.textContent).not.toContain("outline");
+  });
+
+  it("keeps the raw HTML READMEs rely on", () => {
+    const root = document.createElement("div");
+    root.innerHTML = renderMarkdown(
+      '<div align="center"><img src="a.png" width="20"></div>\n\n<details><summary>More</summary>x</details>\n\n- [x] done\n',
+    );
+    expect(root.querySelector("div[align='center'] img")?.getAttribute("width")).toBe("20");
+    expect(root.querySelector("details summary")).not.toBeNull();
+    expect(root.querySelector("input[type='checkbox']")).not.toBeNull();
+  });
+
   it("dedupes heading anchors", () => {
     const root = document.createElement("div");
     root.innerHTML = renderMarkdown("## Same\n\n## Same\n");
diff --git a/src/features/viewers/markdown/render.ts b/src/features/viewers/markdown/render.ts
index 1be1d6334..a08c39dcd 100644
--- a/src/features/viewers/markdown/render.ts
+++ b/src/features/viewers/markdown/render.ts
@@ -81,7 +81,13 @@ export function renderMarkdown(text: string): string {
     }
     html += `<div data-line="${start}">${body}</div>`;
   }
-  return purify().sanitize(html, { USE_PROFILES: { html: true }, ADD_ATTR: ["data-src", "data-line"] });
+  // The preview shares the IDE's document: a `<style>` in a file would restyle the whole IDE.
+  return purify().sanitize(html, {
+    USE_PROFILES: { html: true },
+    ADD_ATTR: ["data-src", "data-line"],
+    FORBID_TAGS: ["style", "form"],
+    FORBID_ATTR: ["style"],
+  });
 }
 
 export type LinkTarget =