> In HTML/CSS you should be able to specify a fallback source if the first returns a non-200.

Except if the HTML/CSS is hosted on that CDN?

DNS didn't fail, and there's nothing you can do in HTML/CS/JS if your CDN fails to serve those things

Content-centric networking had been a central research topic for many years. And many potentially useful systems have been proposed and implemented.

At some point some of them will start to become popular.

Web Browsers should probably retry a different server in DNS if they get a 503 - but they don't.