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 subresource.css to be prefetched.
CORS errors may appear temporarily. You can ignore them.
- the subresource is prefetched using <link> tag in HTML
- the subresource is prefetched using Link HTTP header
- the subresource is prefetched using both methods