diff --git a/packages/player/src/composition-probe.ts b/packages/player/src/composition-probe.ts
index 17de82d86..5ac4c18dd 100644
--- a/packages/player/src/composition-probe.ts
+++ b/packages/player/src/composition-probe.ts
@@ -36,7 +36,15 @@ export interface ProbeCallbacks {
   onRuntimeInjected?: () => void;
 }
 
-function readPositiveDimension(value: string | null): number | null {
+/**
+ * Parse a composition dimension, rejecting anything that isn't a positive
+ * finite number. Exported because the `width`/`height` attribute handlers in
+ * hyperframes-player.ts need the same guard: dimensions feed
+ * scaleIframeToFit's `w / compositionWidth` division, where NaN produces an
+ * invalid `scale(NaN)` transform and zero a division by zero — both render
+ * the player blank with no signal.
+ */
+export function readPositiveDimension(value: string | null): number | null {
   if (value === null) return null;
   const parsed = Number.parseInt(value, 10);
   return Number.isFinite(parsed) && parsed > 0 ? parsed : null;
diff --git a/packages/player/src/hyperframes-player.test.ts b/packages/player/src/hyperframes-player.test.ts
index 6fa8962cc..56e709efa 100644
--- a/packages/player/src/hyperframes-player.test.ts
+++ b/packages/player/src/hyperframes-player.test.ts
@@ -1614,3 +1614,63 @@ describe("HyperframesPlayer playback rate", () => {
     expect(player.playbackRate).toBe(5);
   });
 });
+
+// ── Composition dimension attributes ──
+//
+// width/height feed scaleIframeToFit's `w / compositionWidth` division. A
+// non-numeric, zero, or negative attribute must fall back to the defaults
+// instead of reaching the scale math as NaN (invalid `scale(NaN)` transform)
+// or zero (division by zero) — both blank the player with no signal.
+
+describe("HyperframesPlayer composition dimension attributes", () => {
+  type PlayerWithDimensions = HTMLElement & {
+    _compositionWidth?: number;
+    _compositionHeight?: number;
+  };
+
+  let player: PlayerWithDimensions;
+
+  beforeEach(async () => {
+    await import("./hyperframes-player.js");
+    player = document.createElement("hyperframes-player") as PlayerWithDimensions;
+    document.body.appendChild(player);
+  });
+
+  afterEach(() => {
+    document.body.innerHTML = "";
+  });
+
+  it("applies a valid width and height", () => {
+    player.setAttribute("width", "1280");
+    player.setAttribute("height", "720");
+    expect(player._compositionWidth).toBe(1280);
+    expect(player._compositionHeight).toBe(720);
+  });
+
+  it("falls back to defaults for non-numeric values", () => {
+    player.setAttribute("width", "abc");
+    player.setAttribute("height", "abc");
+    expect(player._compositionWidth).toBe(1920);
+    expect(player._compositionHeight).toBe(1080);
+  });
+
+  it("falls back to defaults for zero", () => {
+    player.setAttribute("width", "0");
+    player.setAttribute("height", "0");
+    expect(player._compositionWidth).toBe(1920);
+    expect(player._compositionHeight).toBe(1080);
+  });
+
+  it("falls back to defaults for negative values", () => {
+    player.setAttribute("width", "-500");
+    player.setAttribute("height", "-500");
+    expect(player._compositionWidth).toBe(1920);
+    expect(player._compositionHeight).toBe(1080);
+  });
+
+  it("recovers the defaults when the attribute is removed", () => {
+    player.setAttribute("width", "1280");
+    player.removeAttribute("width");
+    expect(player._compositionWidth).toBe(1920);
+  });
+});
diff --git a/packages/player/src/hyperframes-player.ts b/packages/player/src/hyperframes-player.ts
index 39f0d2901..81b34f6f0 100644
--- a/packages/player/src/hyperframes-player.ts
+++ b/packages/player/src/hyperframes-player.ts
@@ -1,4 +1,4 @@
-import { CompositionProbe, type ProbeResult } from "./composition-probe.js";
+import { CompositionProbe, type ProbeResult, readPositiveDimension } from "./composition-probe.js";
 import { isControlsClick, setupControls, setupPoster } from "./controls-setup.js";
 import { adoptShadowStyles, createCompositionIframe, scaleIframeToFit } from "./iframe-dom.js";
 import { DirectTimelineClock } from "./direct-timeline-clock.js";
@@ -170,12 +170,16 @@ class HyperframesPlayer extends HTMLElement {
         if (val !== null) this.iframe.srcdoc = prepareSrcdocForElement(this, val);
         else this.iframe.removeAttribute("srcdoc");
         break;
+      // Reject NaN/zero/negative dimensions the same way the composition
+      // probe does (a typo like width="abc" or width="0" would otherwise
+      // reach scaleIframeToFit as scale(NaN) or a division by zero and
+      // blank the player); fall back to the defaults instead.
       case "width":
-        this._compositionWidth = parseInt(val || "1920", 10);
+        this._compositionWidth = readPositiveDimension(val) ?? 1920;
         this._rescale();
         break;
       case "height":
-        this._compositionHeight = parseInt(val || "1080", 10);
+        this._compositionHeight = readPositiveDimension(val) ?? 1080;
         this._rescale();
         break;
       case "controls":
diff --git a/packages/player/src/runtime-message-handler.test.ts b/packages/player/src/runtime-message-handler.test.ts
new file mode 100644
index 000000000..9539c6511
--- /dev/null
+++ b/packages/player/src/runtime-message-handler.test.ts
@@ -0,0 +1,67 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { handleRuntimeMessage, type MessageHandlerCallbacks } from "./runtime-message-handler.js";
+import type { ParentMediaManager } from "./parent-media.js";
+import type { ShaderLoaderState } from "./shader-loader-state.js";
+
+// Only the stage-size branch is exercised here; the rest of the callback
+// surface is satisfied with inert spies so the handler's type contract
+// stays honest without pulling in the real player.
+const makeCallbacks = (): MessageHandlerCallbacks => ({
+  updateControlsTime: vi.fn(),
+  updateControlsPlaying: vi.fn(),
+  dispatchEvent: vi.fn(),
+  seek: vi.fn(),
+  play: vi.fn(),
+  getLoop: vi.fn(() => false),
+  media: { mirrorTime: vi.fn(), promoteToParentProxy: vi.fn() } as unknown as ParentMediaManager,
+  getPlaybackState: vi.fn(() => ({ currentTime: 0, duration: 0, paused: true, lastUpdateMs: 0 })),
+  setPlaybackState: vi.fn(),
+  getShaderLoadingMode: vi.fn(() => "auto"),
+  shaderLoader: { update: vi.fn() } as unknown as ShaderLoaderState,
+  setCompositionSize: vi.fn(),
+  sendControl: vi.fn(),
+  getIframeDoc: vi.fn(() => null),
+});
+
+const stageSizeEvent = (width: unknown, height: unknown, source: object): MessageEvent =>
+  ({
+    source,
+    data: { source: "hf-preview", type: "stage-size", width, height },
+  }) as unknown as MessageEvent;
+
+describe("handleRuntimeMessage stage-size", () => {
+  it("applies a finite positive stage size", () => {
+    const frameWindow = {} as Window;
+    const callbacks = makeCallbacks();
+
+    handleRuntimeMessage(stageSizeEvent(1280, 720, frameWindow), frameWindow, callbacks);
+
+    expect(callbacks.setCompositionSize).toHaveBeenCalledWith(1280, 720);
+  });
+
+  it.each([
+    ["Infinity width", Infinity, 720],
+    ["Infinity height", 1280, Infinity],
+    ["NaN width", NaN, 720],
+    ["zero width", 0, 720],
+    ["negative height", 1280, -720],
+    ["string width", "1280", 720],
+  ])("ignores stage-size with %s", (_label, width, height) => {
+    const frameWindow = {} as Window;
+    const callbacks = makeCallbacks();
+
+    handleRuntimeMessage(stageSizeEvent(width, height, frameWindow), frameWindow, callbacks);
+
+    expect(callbacks.setCompositionSize).not.toHaveBeenCalled();
+  });
+
+  it("ignores messages from a different source window", () => {
+    const frameWindow = {} as Window;
+    const callbacks = makeCallbacks();
+
+    handleRuntimeMessage(stageSizeEvent(1280, 720, {}), frameWindow, callbacks);
+
+    expect(callbacks.setCompositionSize).not.toHaveBeenCalled();
+  });
+});
diff --git a/packages/player/src/runtime-message-handler.ts b/packages/player/src/runtime-message-handler.ts
index 3fb42f2dc..42deff886 100644
--- a/packages/player/src/runtime-message-handler.ts
+++ b/packages/player/src/runtime-message-handler.ts
@@ -86,7 +86,11 @@ export function handleRuntimeMessage(
 
   if (
     data["type"] === "stage-size" &&
+    // Finite-check like the timeline branch above: `> 0` alone lets
+    // Infinity through, which scales the iframe to 0 and blanks it.
+    Number.isFinite(data["width"]) &&
     (data["width"] as number) > 0 &&
+    Number.isFinite(data["height"]) &&
     (data["height"] as number) > 0
   ) {
     callbacks.setCompositionSize(data["width"] as number, data["height"] as number);