Am I online? The answer flips as you watch

Both, and this is the exact case the indicator is weakest in. The browser is answering a narrow question — is there a network interface with a route — and a hotel or airport portal waiting for you to accept its terms answers yes to that while blocking everything else. So does a router whose own upstream has died, and so does a home network where the modem is up and the provider is down. If the banner is green and a page will not load, the fault is past your router, and no event-based indicator anywhere can see past your router.

The absence of any usable network interface, as the operating system sees it — which is a lower bar than it sounds. Pulling the Ethernet cable on a machine with no other adapter does it. Switching off the only Wi-Fi radio does it. What does not do it is losing internet access while an interface remains up, and a surprising number of machines always have one up: a VPN client left running, a virtual switch from a hypervisor, or the adapter a container platform installs all count as interfaces, and they can hold the answer at online while the physical port is unplugged.

Because the Network Information API is implemented only in Chromium-based browsers — Chrome, Edge, Opera and Samsung Internet. Firefox and Safari have both declined to ship it, largely because effective type, downlink and round-trip estimates add fingerprinting surface for a benefit sites can obtain more honestly by measuring their own performance. The panel says so directly instead of drawing empty rows, and the indicator itself works everywhere, since online and offline events are supported by every browser in current use.

Because that is the highest number the API will ever report. The value is capped at 10 Mbit/s and rounded to the nearest 25 kbit/s deliberately, so that a page cannot use your exact throughput as an identifier, and it is an estimate from recent traffic rather than a measurement of your line. A fiber connection, a fast cable connection and a good office link all report the same ceiling, which makes the field useful for deciding whether to load a large image and useless for comparing plans.

Because the label describes recent performance, not the physical medium. The browser sorts what it has been observing into four buckets by round-trip time and throughput, so a fiber line reached through a congested wireless radio can be classified as 3g entirely correctly, and a well-behaved mobile connection is often reported as 4g. The value also lags: a tab that has been idle reports the conditions it last saw, and it settles again after a little traffic.

Yes, and that is a side effect of how it is built rather than a feature that had to be added. The page is a static file your browser already has, and the indicator depends on events from the operating system rather than on reaching a server, so the moment you disconnect it carries on running and records the disconnection. Nothing on it needs the network in order to tell you the network is gone.

No — this page opens no connection of its own at any point. The banner, the timer and the log are produced from events and clock readings inside the tab, and the drop times you see are never transmitted anywhere. It is the one page in this section that measures nothing, which is precisely what lets it answer instantly.