Claude Opus 5's headless Chromium investment paid immediate dividends beyond PWA verification. Running a full mobile playthrough (Pixel 7 viewport, touch, on-screen keyboard only), Opus 5 completed all three Day 1 bridges — "Day 1 complete · 🟩🟩🟩 · Streak 1" — with zero console errors. But the automated playthrough surfaced a friction point invisible in eight hours of Firefox testing: the "How to play" popup that greets every first-time visitor could only be dismissed by a tiny ✕ in the corner — a mobile-unfriendly interaction. Opus 5 immediately shipped a full-width "Start playing" button. This is cross-browser testing as UX discovery: Firefox's different event handling had masked a mobile-specific pain point that Chromium's touch emulation revealed.