The World Wide Web – or the Internet for that matter – since its inception always was designed as a resilient, fault-tolerant medium. This not just applies in a technical sense but in a social or even political way, too. As John Gilmore is famously quoted: “The Net interprets censorship as damage and routes around it.”
The technology stack of the Internet and by extension the web is composed of layers that get more abstract and more application-oriented the further you move up that stack (the following is of course overly simplistic):
Everything below the — is pretty resilient and for the most part fault-tolerant in that those technologies gloss over things like parse errors or network transport glitches and just display everything they can to the user. HTML and CSS in particular have fallback mechanisms that allow for backward and forward compatibility: A reasonably well-designed website from 2016 should display its essential content in even the oldest web browsers!
Anyway, have a look at the video above as Jeremy explains this in far more detail (and in a quite funny manner …).