An interesting thought experiment from Chris: if a client asked you to build a website that could last a minimum of 10 years, what would you do? Lots of influential people in his answers, but I'm most interested in some of the proposed solutions. Basically:

• Use semantic HTML and CSS; minify JS as much as possible (and obviously no JS frameworks or dependencies);
• No externally hosted resources or data i.e. you can't call out. Either inline, self-host, or remove everything external;
• Clean, minimal design; simple colour palette; good typography (several people recommend system fonts, but I actually feel it would be best to host font files locally and fall back to "classic" system fonts. You never know when Windows might kill Arial, or when a new OS suddenly takes off);
• Consider internationalisation support (who says English is the main language in 10 years?);
• Stick to viewport units wherever possible to adapt to new screen resolutions and sizes; ditto SVG;
  
• Minimal copy – "even words go stale";
• Register the domain and hosting for the full 10 years upfront and set up auto-renewal of stuff like SSL.
  

Based on experience and observation in my time in the industry, I’d say it’s somewhere around 75% of websites are completely gone after 10 years, let alone particular URL’s on those websites being exactly the same (reliable external resources).