Critical CSS generation timed out occurs when WP Rocket fails to retrieve the page needed to build above‑the‑fold styles. The service attempts to request the URL, but the request is blocked, times out, or returns an unexpected response. When the request does not deliver valid HTML, the generator stops and WP Rocket shows an error.
The issue usually stems from server restrictions, firewall rules, or missing cron execution. It also appears when other optimization plugins alter the output before the generator runs.