diff --git a/docs/reference/lint-rules.mdx b/docs/reference/lint-rules.mdx
index e1ab7d70439..b8aad8358d4 100644
--- a/docs/reference/lint-rules.mdx
+++ b/docs/reference/lint-rules.mdx
@@ -47,6 +47,12 @@ lints the file it is editing.
 
 **Fix:** Give every caption element the same `data-track-index`.
 
+### `multiple_scene_templates`
+
+**Warning.** A file has more than one `<template>` holding a composition root and no composition root outside them. Only the first one renders; the message names each template.
+
+**Fix:** Keep one composition root per file, or move the other scenes into their own files.
+
 ### `clip_ends_past_root_duration`
 
 **Warning.** A clip runs past the root composition's `data-duration`, so it is cut off in previews, posters and renders.
diff --git a/packages/core/src/compiler/htmlBundler.test.ts b/packages/core/src/compiler/htmlBundler.test.ts
index 7011c39482a..00e1570ab19 100644
--- a/packages/core/src/compiler/htmlBundler.test.ts
+++ b/packages/core/src/compiler/htmlBundler.test.ts
@@ -285,6 +285,53 @@ describe("bundleToSingleHtml", () => {
     },
   );
 
+  it("bundles the first scene template of a file with two scene templates and no live root", async () => {
+    const dir = makeTempProject({
+      "index.html": `<!doctype html><html><body>
+        <div data-composition-id="root" data-width="320" data-height="180" data-duration="6">
+          <div data-composition-id="scene" data-composition-src="compositions/scene.html" data-start="0" data-duration="6"></div>
+        </div>
+      </body></html>`,
+      "compositions/scene.html": `<template><div data-composition-id="scene" data-duration="6"><h1>first scene</h1></div></template>
+        <template><div data-composition-id="other"><h1>second scene</h1></div></template>`,
+    });
+
+    const bundled = await bundleToSingleHtml(dir, { entryFile: "compositions/scene.html" });
+
+    expect(bundled).toContain("first scene");
+    expect(bundled).not.toContain("second scene");
+  });
+
+  it.each([
+    [
+      "a card template before the scene",
+      `<template id="card"><div class="card">card</div></template>`,
+      "mounted scene",
+      "",
+    ],
+    [
+      "two scene templates",
+      "",
+      "mounted scene",
+      `<template><div data-composition-id="other"><h1>second scene</h1></div></template>`,
+    ],
+  ])("mounts the scene template of a file with %s", async (_label, before, expected, after) => {
+    const dir = makeTempProject({
+      "index.html": `<!doctype html><html><body>
+        <div data-composition-id="root" data-width="320" data-height="180" data-duration="6">
+          <div data-composition-id="scene" data-composition-src="compositions/scene.html" data-start="0" data-duration="6"></div>
+        </div>
+      </body></html>`,
+      "compositions/scene.html": `${before}<template><div data-composition-id="scene" data-duration="6"><h1>mounted scene</h1></div></template>${after}`,
+    });
+
+    const bundled = await bundleToSingleHtml(dir);
+
+    expect(bundled).toContain(expected);
+    expect(bundled).not.toContain("second scene");
+    expect(bundled).not.toContain(">card<");
+  });
+
   it("rebases direct-entry authored asset paths before inlining", async () => {
     const spriteSvg = '<svg xmlns="http://www.w3.org/2000/svg"><circle r="4"/></svg>';
     const bgSvg = '<svg xmlns="http://www.w3.org/2000/svg"><rect width="8" height="8"/></svg>';
diff --git a/packages/core/src/compiler/htmlDocument.ts b/packages/core/src/compiler/htmlDocument.ts
index 45fe3fdca99..94ae0ae952e 100644
--- a/packages/core/src/compiler/htmlDocument.ts
+++ b/packages/core/src/compiler/htmlDocument.ts
@@ -1,4 +1,5 @@
 import { acceleratedCanvasTrackerSource } from "../pageMotionBlur.js";
+import { findSceneTemplate } from "@hyperframes/parsers/sub-composition-validity";
 import { parseHTML } from "linkedom";
 
 export const RUNTIME_BOOTSTRAP_ATTR = "data-hyperframes-preview-runtime";
@@ -16,8 +17,30 @@ const SIMPLE_RUNTIME_FLAG_ASSIGNMENTS = [
 
 const LEADING_COMMENTS = /^(?:\s|<!--(?:>|->|[\s\S]*?-->))*/;
 
+export interface SceneRootLookup {
+  root: Element | null;
+  hasTemplate: boolean;
+  hasLiveRoot: boolean;
+}
+
+export function locateSceneRoot(html: string): SceneRootLookup {
+  const { document } = parseHTML(html);
+  const liveRoot = document.querySelector("[data-composition-id]") as Element | null;
+  const templates = Array.from(document.querySelectorAll("template"));
+  const sceneRoot =
+    (findSceneTemplate(templates)?.content.querySelector(
+      "[data-composition-id]",
+    ) as Element | null) ?? null;
+  return {
+    root: sceneRoot,
+    hasTemplate: templates.length > 0,
+    hasLiveRoot: liveRoot !== null,
+  };
+}
+
 export function isTemplateEntry(html: string): boolean {
-  return /^<template[\s>/]/i.test(html.replace(LEADING_COMMENTS, ""));
+  const { root, hasTemplate, hasLiveRoot } = locateSceneRoot(html);
+  return hasTemplate && root !== null && !hasLiveRoot;
 }
 
 export function isFullHtmlDocument(html: string): boolean {
diff --git a/packages/core/src/compiler/standaloneEntry.test.ts b/packages/core/src/compiler/standaloneEntry.test.ts
index bb8e7e10f52..62a406f153d 100644
--- a/packages/core/src/compiler/standaloneEntry.test.ts
+++ b/packages/core/src/compiler/standaloneEntry.test.ts
@@ -1,25 +1,96 @@
 import { parseHTML } from "linkedom";
 import { describe, expect, it } from "vitest";
+import { locateSceneRoot } from "./htmlDocument";
 import { extractStandaloneEntryFromIndex, isTemplateEntry } from "./standaloneEntry";
 
+const ROOT = `<div id="root" data-composition-id="scene" data-width="640" data-height="360" data-duration="3"><p id="label">Hello</p></div>`;
+const SCENE_SCRIPT = `<script>window.__timelines = window.__timelines || {}; const tl = gsap.timeline({ paused: true }); tl.to("#label", { opacity: 1, duration: 3 }, 0); window.__timelines["scene"] = tl;</script>`;
+const SCENE_STYLE = `<style>#root { position: absolute; inset: 0; background: #123456; }</style>`;
+
 describe("isTemplateEntry", () => {
   it.each([
-    "<template></template>",
-    " \n<!-- Scene author notes -->\n<template id='scene'></template>",
-    "<!-- a --><!-- <template> is inert --><TEMPLATE></TEMPLATE>",
-    "<!--><template></template>",
-  ])("recognizes a template entry past leading comments: %s", (html) => {
+    `<template>${ROOT}</template>`,
+    ` \n<!-- Scene author notes -->\n<template id='scene'>${ROOT}</template>`,
+    `<!-- a --><!-- <template> is inert --><TEMPLATE>${ROOT}</TEMPLATE>`,
+    `<!--><template>${ROOT}</template>`,
+  ])("recognizes a template-first fragment entry: %s", (html) => {
+    expect(isTemplateEntry(html)).toBe(true);
+  });
+
+  it("recognizes the full-document shape whose composition root lives in the body template", () => {
+    const html = `<!doctype html>
+<html>
+  <head><meta charset="UTF-8" /></head>
+  <body>
+    <template>
+      ${SCENE_STYLE}
+      ${ROOT}
+      ${SCENE_SCRIPT}
+    </template>
+  </body>
+</html>`;
     expect(isTemplateEntry(html)).toBe(true);
   });
 
   it.each([
     "<template-card></template-card>",
-    "<!doctype html><html><body><template></template></body></html>",
-    "<div></div><template></template>",
-    "<!-- <template></template> -->",
+    `<template-card>${ROOT}</template-card>`,
+    `<!doctype html><html><body><template></template></body></html>`,
+    `<div></div><template></template>`,
+    `<!-- <template>${ROOT}</template> -->`,
+    `<!doctype html><html><body>${ROOT}</body></html>`,
   ])("does not mistake other entries for a template scene: %s", (html) => {
     expect(isTemplateEntry(html)).toBe(false);
   });
+
+  it("keeps a full document with a live composition root outside the template a plain entry", () => {
+    const html = `<!doctype html>
+<html>
+  <body>
+    <div id="live" data-composition-id="live" data-width="640" data-height="360" data-duration="3"></div>
+    <template>${ROOT}</template>
+  </body>
+</html>`;
+    expect(isTemplateEntry(html)).toBe(false);
+  });
+
+  it("keeps a template-first fragment with a live composition root beside it a plain entry", () => {
+    const html = `<template>${ROOT}</template><div data-composition-id="live"></div>`;
+    expect(isTemplateEntry(html)).toBe(false);
+  });
+
+  it.each([
+    `<template id="card"><div class="card">Card</div></template><template id="scene">${ROOT}</template>`,
+    `<!doctype html><html><body><template><svg><symbol id="i"></symbol></svg></template><template>${ROOT}</template></body></html>`,
+    `<template></template><template><p>x</p></template><template>${ROOT}</template>`,
+  ])("finds the scene template when an earlier template holds no composition root: %s", (html) => {
+    expect(isTemplateEntry(html)).toBe(true);
+    expect(locateSceneRoot(html).root?.getAttribute("data-composition-id")).toBe("scene");
+  });
+
+  it("renders the first scene template when several hold a composition root and none is live", () => {
+    const html = `<template id="intro">${ROOT}</template><template><div data-composition-id="later"></div></template>`;
+    expect(locateSceneRoot(html).root?.getAttribute("data-composition-id")).toBe("scene");
+    expect(isTemplateEntry(html)).toBe(true);
+  });
+
+  it("does not refuse when a live root makes the file a plain entry", () => {
+    const html = `<template>${ROOT}</template><template>${ROOT}</template><div data-composition-id="live"></div>`;
+    expect(isTemplateEntry(html)).toBe(false);
+  });
+
+  it("reports no root when no template holds a composition root", () => {
+    const lookup = locateSceneRoot(`<template><p>a</p></template><template><p>b</p></template>`);
+    expect(lookup).toMatchObject({ root: null, hasTemplate: true, hasLiveRoot: false });
+  });
+
+  it("reads a single template and a template-free document as before", () => {
+    expect(locateSceneRoot(`<template>${ROOT}</template>`).root?.id).toBe("root");
+    expect(locateSceneRoot(`<!doctype html><html><body>${ROOT}</body></html>`)).toMatchObject({
+      hasTemplate: false,
+      hasLiveRoot: true,
+    });
+  });
 });
 
 describe("extractStandaloneEntryFromIndex", () => {
diff --git a/packages/core/src/compiler/standaloneEntry.ts b/packages/core/src/compiler/standaloneEntry.ts
index 984979c4f1b..3bd46c8e04b 100644
--- a/packages/core/src/compiler/standaloneEntry.ts
+++ b/packages/core/src/compiler/standaloneEntry.ts
@@ -1,5 +1,6 @@
 import { parseHTML } from "linkedom";
 import { HF_AUDIO_FX_ATTR } from "../audioFx";
+import { locateSceneRoot } from "./htmlDocument";
 
 export { isTemplateEntry } from "./htmlDocument";
 
@@ -7,13 +8,8 @@ function normalizeCompositionSrcPath(srcPath: string): string {
   return srcPath.replace(/\\/g, "/").replace(/^\.\//, "");
 }
 
-// linkedom has no inert <template> content, so the template's innerHTML is re-parsed.
 function readSceneRoot(entryHtml: string | undefined): Element | null {
-  if (!entryHtml) return null;
-  const { document } = parseHTML(entryHtml);
-  const template = document.querySelector("template");
-  const scope = template ? parseHTML(template.innerHTML).document : document;
-  return scope.querySelector("[data-composition-id]") as Element | null;
+  return entryHtml ? locateSceneRoot(entryHtml).root : null;
 }
 
 // The scene's own chain replaces the index's, which belongs to the whole project.
diff --git a/packages/core/src/runtime/compositionLoader.test.ts b/packages/core/src/runtime/compositionLoader.test.ts
index 261aa6507c4..00977798b2a 100644
--- a/packages/core/src/runtime/compositionLoader.test.ts
+++ b/packages/core/src/runtime/compositionLoader.test.ts
@@ -149,6 +149,27 @@ describe("loadCompositions external hosts", () => {
     ).toBe(false);
   });
 
+  it.each([
+    ["a card template before the scene", `<template><div class="card">card</div></template>`, ""],
+    [
+      "two scene templates",
+      "",
+      `<template><div data-composition-id="other"><p>second scene</p></div></template>`,
+    ],
+  ])("mounts the scene template of a fetched file with %s", async (_label, before, after) => {
+    const host = appendExternalHost("https://example.com/comp.html", "scene-1");
+    const scene = `<template><div data-composition-id="scene-1"><p>Hello World</p></div></template>`;
+    vi.spyOn(globalThis, "fetch").mockResolvedValue(
+      new Response(`${before}${scene}${after}`, { status: 200 }),
+    );
+
+    await loadFixture();
+
+    expect(host.textContent).toContain("Hello World");
+    expect(host.textContent).not.toContain("card");
+    expect(host.textContent).not.toContain("second scene");
+  });
+
   it("sizes a flattened inner root from a px-suffixed or fractional size", async () => {
     const host = appendExternalHost("https://example.com/comp.html", "scene-1");
     const compositionHtml = `
diff --git a/packages/core/src/runtime/compositionLoader.ts b/packages/core/src/runtime/compositionLoader.ts
index 256b5b8a945..cc1ee698a31 100644
--- a/packages/core/src/runtime/compositionLoader.ts
+++ b/packages/core/src/runtime/compositionLoader.ts
@@ -1,3 +1,4 @@
+import { findSceneTemplate } from "@hyperframes/parsers/sub-composition-validity";
 import { wallClearTimeout, wallSetTimeout } from "./wallClock";
 import {
   SVG_REFERENCE_ALIASES_ATTR,
@@ -186,6 +187,11 @@ function rewriteSubCompositionAssetPaths(root: ParentNode, compositionUrl: URL |
   }
 }
 
+function sceneOrFirstTemplate(doc: Document): HTMLTemplateElement | null {
+  const templates = Array.from(doc.querySelectorAll<HTMLTemplateElement>("template"));
+  return findSceneTemplate(templates) ?? templates[0] ?? null;
+}
+
 function uniqueCompositionId(baseId: string, index: number): string {
   return `${baseId}__hf${index}`;
 }
@@ -801,7 +807,7 @@ async function mountExternalCompositions(
             ? doc.querySelector<HTMLTemplateElement>(
                 `template#${CSS.escape(authoredCompositionId)}-template`,
               )
-            : null) ?? doc.querySelector<HTMLTemplateElement>("template");
+            : null) ?? sceneOrFirstTemplate(doc);
         const sourceNode = template ? template.content : doc.body;
         return await mount({
           host,
diff --git a/packages/lint/src/rules/composition.test.ts b/packages/lint/src/rules/composition.test.ts
index dbf984409c3..97a1ca6ea28 100644
--- a/packages/lint/src/rules/composition.test.ts
+++ b/packages/lint/src/rules/composition.test.ts
@@ -4,6 +4,32 @@ import { lintHyperframeHtml } from "../hyperframeLinter.js";
 import { parseCompositionVariables } from "@hyperframes/parsers/composition";
 
 describe("composition rules", () => {
+  describe("multiple_scene_templates", () => {
+    const scene = (id: string) =>
+      `<div data-composition-id="${id}" data-width="320" data-height="180"></div>`;
+    const find = async (html: string) =>
+      (await lintHyperframeHtml(html, { isSubComposition: true })).findings.find(
+        (f) => f.code === "multiple_scene_templates",
+      );
+
+    it("warns, naming each scene template", async () => {
+      const finding = await find(
+        `<template id="intro">${scene("a")}</template><template>${scene("b")}</template>`,
+      );
+      expect(finding?.severity).toBe("warning");
+      expect(finding?.message).toContain('template #1 (id="intro") holding composition "a"');
+      expect(finding?.message).toContain('template #2 holding composition "b"');
+    });
+
+    it.each([
+      `<template>${scene("a")}</template>`,
+      `<template><svg></svg></template><template>${scene("a")}</template>`,
+      `<template>${scene("a")}</template><template>${scene("b")}</template>${scene("live")}`,
+    ])("is quiet for %s", async (html) => {
+      expect(await find(html)).toBeUndefined();
+    });
+  });
+
   describe("canonical timing contract", () => {
     it("rejects deprecated attributes even when canonical attributes are also present", async () => {
       const result = await lintHyperframeHtml(`<!doctype html><html><body>
diff --git a/packages/lint/src/rules/composition.ts b/packages/lint/src/rules/composition.ts
index b9f2be48002..dae349bb929 100644
--- a/packages/lint/src/rules/composition.ts
+++ b/packages/lint/src/rules/composition.ts
@@ -1,5 +1,6 @@
 import type { LintContext, HyperframeLintFinding, ExtractedBlock, OpenTag } from "../context";
 import selectorParser from "postcss-selector-parser";
+import { parseHTML } from "linkedom";
 import {
   findHtmlTag,
   extractTimelineRegistryKeys,
@@ -1946,6 +1947,32 @@ export const compositionRules: Array<(ctx: LintContext) => HyperframeLintFinding
         snippet: truncateSnippet(style.content.slice(0, 120)),
       }));
   },
+
+  // multiple_scene_templates: the renderer takes the first scene template of a file with no live root.
+  ({ rawSource }) => {
+    const { document } = parseHTML(rawSource);
+    if (document.querySelector("[data-composition-id]")) return [];
+    const scenes = Array.from(document.querySelectorAll("template")).flatMap((template, index) => {
+      const compositionId = parseHTML(template.innerHTML)
+        .document.querySelector("[data-composition-id]")
+        ?.getAttribute("data-composition-id");
+      if (compositionId === undefined) return [];
+      const id = template.getAttribute("id");
+      return [
+        `template #${index + 1}${id ? ` (id="${id}")` : ""} holding composition "${compositionId}"`,
+      ];
+    });
+    if (scenes.length < 2) return [];
+    return [
+      {
+        code: "multiple_scene_templates",
+        severity: "warning",
+        message: `This file has more than one scene template and no live composition root, so only the first renders: ${scenes.join("; ")}.`,
+        fixHint:
+          "Keep one composition root per file, or move the other scenes into their own files.",
+      },
+    ];
+  },
 ];
 
 const CLIP_MEDIA_TAGS = new Set<string>(["img", "video", "audio"]);
diff --git a/packages/parsers/src/subCompositionValidity.ts b/packages/parsers/src/subCompositionValidity.ts
index 9f64eb17871..c4232237675 100644
--- a/packages/parsers/src/subCompositionValidity.ts
+++ b/packages/parsers/src/subCompositionValidity.ts
@@ -55,11 +55,25 @@ export interface ParsableDocumentLike {
   documentElement: { outerHTML?: string } | null;
   body?: { innerHTML?: string | null } | null;
   querySelector(selector: string): { innerHTML?: string | null } | null;
+  querySelectorAll(selector: string): ArrayLike<{ innerHTML?: string | null } & SceneTemplateLike>;
+}
+
+export interface SceneTemplateLike {
+  content: { querySelector(selector: string): unknown };
+}
+
+export function findSceneTemplate<T extends SceneTemplateLike>(
+  templates: ArrayLike<T>,
+): T | undefined {
+  return Array.from(templates).find(
+    (template) => template.content.querySelector("[data-composition-id]") != null,
+  );
 }
 
 /** Resolve authored content without invoking linkedom's synthetic body getter. */
 export function resolveSubCompositionContent(html: string, document: ParsableDocumentLike) {
-  const template = document.querySelector("template");
+  const templates = Array.from(document.querySelectorAll("template"));
+  const template = findSceneTemplate(templates) ?? templates[0];
   if (template) return { contentHtml: template.innerHTML ?? "", hasTemplate: true };
 
   const body = document.querySelector("body");
diff --git a/packages/producer/src/services/render/stages/compileStage.test.ts b/packages/producer/src/services/render/stages/compileStage.test.ts
index 4aa158509b7..74ed8487db4 100644
--- a/packages/producer/src/services/render/stages/compileStage.test.ts
+++ b/packages/producer/src/services/render/stages/compileStage.test.ts
@@ -521,6 +521,78 @@ describe("resolveEntryHtmlPath", () => {
     expect(html).not.toContain('id="other"');
   });
 
+  describe("canonical sub-composition document", () => {
+    const SCENE_ROOT = `<div id="root" data-composition-id="scene" data-width="640" data-height="360" data-duration="3"><p id="label">Hello</p></div>`;
+    const SCENE_SCRIPT = `<script>window.__timelines = window.__timelines || {}; const tl = gsap.timeline({ paused: true }); tl.to("#label", { opacity: 1, duration: 3 }, 0); window.__timelines["scene"] = tl;</script>`;
+    const SCENE_STYLE = `<style>#root { position: absolute; inset: 0; background: #123456; }</style>`;
+    const INDEX = `<!doctype html><html><body>
+        <div data-composition-id="root" data-width="640" data-height="360" data-duration="9">
+          <div id="scene" data-composition-id="scene" data-composition-src="compositions/scene.html"></div>
+        </div>
+      </body></html>`;
+
+    function writeScene(html: string): void {
+      if (!fixture) throw new Error("fixture not set up");
+      mkdirSync(join(fixture.workDir, "project", "compositions"));
+      writeFileSync(join(fixture.workDir, "project", "compositions", "scene.html"), html);
+    }
+
+    it("isolates a full-document scene whose root lives in the body template", () => {
+      fixture = setupFixture(INDEX);
+      writeScene(`<!doctype html>
+<html>
+  <head><meta charset="UTF-8" /></head>
+  <body>
+    <template>
+      ${SCENE_STYLE}
+      ${SCENE_ROOT}
+      ${SCENE_SCRIPT}
+    </template>
+  </body>
+</html>`);
+      const wrapper = resolve("compositions/scene.html");
+      expect(wrapper).toBe(join(fixture.workDir, "standalone-entry.html"));
+      const html = readFileSync(wrapper, "utf-8");
+      expect(html).toContain('id="scene"');
+      expect(html).toContain('data-duration="3"');
+      expect(html).not.toContain('data-duration="9"');
+    });
+
+    it("still isolates the template-first fragment form of the same scene", () => {
+      fixture = setupFixture(INDEX);
+      writeScene(`<template>${SCENE_STYLE}${SCENE_ROOT}${SCENE_SCRIPT}</template>`);
+      const wrapper = resolve("compositions/scene.html");
+      expect(wrapper).toBe(join(fixture.workDir, "standalone-entry.html"));
+      expect(readFileSync(wrapper, "utf-8")).toContain('data-duration="3"');
+    });
+
+    it("isolates the first scene template when a file holds two and no live root", () => {
+      fixture = setupFixture(INDEX);
+      writeScene(
+        `<template>${SCENE_ROOT}</template><template><div data-composition-id="other" data-duration="5"></div></template>`,
+      );
+      const wrapper = resolve("compositions/scene.html");
+      expect(wrapper).toBe(join(fixture.workDir, "standalone-entry.html"));
+      const html = readFileSync(wrapper, "utf-8");
+      expect(html).toContain('data-duration="3"');
+      expect(html).not.toContain('id="other"');
+    });
+
+    it("leaves a full document with a live root outside its template untouched", () => {
+      fixture = setupFixture(INDEX);
+      writeScene(`<!doctype html>
+<html>
+  <body>
+    <div id="live" data-composition-id="live" data-width="640" data-height="360" data-duration="3"></div>
+    <template>${SCENE_ROOT}</template>
+  </body>
+</html>`);
+      expect(resolve("compositions/scene.html")).toBe(
+        join(fixture.workDir, "project", "compositions", "scene.html"),
+      );
+    });
+  });
+
   it("refuses a template entry that index.html does not mount", () => {
     fixture = setupFixture(PLAIN_HTML);
     mkdirSync(join(fixture.workDir, "project", "compositions"));