The examples are practically invisible in dark mode. The author should use rasterized screenshots with white backgrounds, or a media query with a brighter color.
Web Developers shouldn't need to "support" particular browsers at all, that's what standards are for. Nobody should be "supporting" any non-standard functionality at all.
What solution is there here? No one would suggest that letting every site set up a long-lived network connection would be a good idea. So you need a central trustworthy connection. If you don't trust the company that makes your browser/OS then why are you using it?
> you should be able to just schedule a web notification locally. But you can't, you can only subscribe to a server.
This isn't entirely true; WebPush is server only, but the Notification API can be used locally. You could schedule a timeout that causes a notification, as long as the page stays open, and to do it while the page is closed you could use a ServiceWorker. Potentially a ServiceWorker may or may not stay active depending on the user's battery settings, but native notifications too can also be disabled by the user.
I've owned a few EufyCams and a couple doorbell cams. They work pretty well; the motion sensing is a little too sensitive for an urban area, but they'd be perfect for suburban/rural areas. They can detect people, cars, and animals. The newest base station can be upgraded with a SATA HDD/SSD.
There was a security kerfuffle last year because motion notifications sent to your phone contained video previews which were then stored insecurely. They fixed that, and it's a feature you can disable. It's not nearly as bad as what has happened to Nest/Wyze cams where attackers can gain access to live video and even talk through the speakers.