Another great post from Tim. The sooner we recognize how little we actually control when it comes to our websites, the better we’ll be able to serve our users.
Getting a website successfully delivered to a visitor depends on a series of actions. My server must spit something out. That something must be passed over some network. That something must then be consumed by another something: some client (often a browser) on some device. Finally, the visitor views that something in whatever context they happen to be in.
There are a lot of unpredictable layers here.
See also (from this site):