Each probe is a HEAD request with a three-second timeout that follows no redirects. Up means the host answered with anything below 500 — including 403 and 405, which many sites deliberately return to datacenter requests; a response is still a response, and treating it as an outage would produce false alarms. Degraded means a 5xx answer: reachable, but the origin is erroring, which is the classic partially-down state. Down means nothing came back at all — timeout, DNS failure or refused connection, which are indistinguishable to the caller and all count as unreachable.
The probe runs from DevBench's servers, so it answers "is this service reachable from the internet?" — not "is my connection working?". If every card is green but one site fails for you, look local first: your ISP, DNS resolver, VPN or the site blocking your network. If a service flips between Degraded and Up across a few checks, an incident is likely in progress — re-check in a minute to watch it settle. Results cache in this browser only, and the request allowlist is fixed: the tool probes exactly these well-known public hostnames and nothing else.
Also useful: DNS Lookup, IP Info, and HTTP Status Reference.