Wait for the SXG cache to fill (up to 1 minute, typically 15 seconds). Continuing without waiting won't break anything, but keep in mind results may not be final and you may need to repeat the next step one or more times.
Click the same button again.
Observe CORS error when the browser tries to prefetch one of the absent.css subresources (the one with "tests/worker" in the URL).
Explanation
The problematic subresource doesn't set Vary header, uses Cloudflare cache and Cloudflare worker.
This combination causes Cloudflare ASX to bind the main document to inaccessible subresource.