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 unstable.css.
Explanation
The problematic subresource has a HTTP header named X-Unstable-Header. This header has different value on each request.