If you've seen this page pop up when installing your new cable modem, you can get stuck in an endless loop of frustration.
The Comcast/Xfinity walled garden, gstatic 204, captive portal, endless loop, signup redirect, certificate error—or whatever else you want to call this mess—is a real pain. This scenario applies especially to anyone who owns their own equipment, typically a cable modem and separate router. For example, you may have a Motorola cable modem and a TP-Link wireless router. Basically, any combination of new or upgraded gear can trigger the problem.