Sites which were carefully designed are not broken, and I think that is a good starting point when designing with longevity in mind.
Five hundred years is a long time, but I think it's reasonable to try to design a site that could last, for example, 25 years, because you can already write something which COULD HAVE worked for the PREVIOUS 25 years by testing with older browsers.
You'll want to restrict yourself to a subset of HTML which is supported by all of them, perhaps with some progressive enhancement.
Five hundred years is a long time, but I think it's reasonable to try to design a site that could last, for example, 25 years, because you can already write something which COULD HAVE worked for the PREVIOUS 25 years by testing with older browsers.
You'll want to restrict yourself to a subset of HTML which is supported by all of them, perhaps with some progressive enhancement.