Subresource HTTP/2 prioritization impact on SXG demo
Open Network tab in Chrome Dev Tools.
Click the "Prefetch" button.
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 errors when the browser tries to prefetch cache-on-worker-off.jpg subresources.
Click the "Populate cache" button, wait 1 second or more, and then hit "Prefetch" button.
Wait for the SXG cache to fill.
Click the "Prefetch" button again.
Observe no CORS errors.
Explanation
The problematic subresource mutates by including "Cf-Bgj" header for second and next requests. Also it uses Cloudflare cache and doesn't use Cloudflare worker.
This combination causes Cloudflare ASX to bind the main document to inaccessible subresource.
By hitting "Populate cache" you make sure the "Cf-Bgj" header is always there which guarantees the subresource doesn't mutate.