The render engine

What render.mjs does that the article's version doesn't

• Network sandbox: every request is served from disk under a fake origin, and anything else is blocked and fails the render.
• Parallel workers: six pages render interleaved frames. That's only valid because seek(t) is pure. Proof: --workers 1 and --workers 6 gave 720/720 identical frame hashes.
• Consistent Chromium: forced sRGB, no font hinting, no LCD text, CPU rasterization.
• Ready signal: a window.filmReady handshake after explicit document.fonts.load(). fonts.ready alone misses fonts that only a canvas uses.
• Exclusive end frame: loop-correct by construction.

Four bugs found in audit and fixed

1. new URL(import.meta.url).pathname keeps %20, which breaks paths with spaces. Fix: fileURLToPath.
2. p.startsWith(ROOT) lets motion-studio-secrets/ through. Fix: ROOT + path.sep.
3. Color. -colorspace bt709 only tags the file; the RGB→YUV math is a separate step. Test: on ffmpeg 6.1, #10b981 decoded as (0, 163, 126), visibly duller. ffmpeg 9 was fine. Fix: explicit scale=out_color_matrix=bt709, plus setparams (Claude found PNG frames arrive tagged "unspecified").
4. Page errors kept rendering broken frames. Fix: fail fast with the frame time.

Fact-check of the article

• ⚠️ tmix with 4 subframes is a 270° shutter, looking forward from each frame time; film convention is 180°.
• ⚠️ The article's spring treats overdamped as critical (see Lesson 6).
• ✅ The Remotion and HyperFrames commands match their docs. HyperFrames uses Puppeteer, not Playwright.