There's also the fact that human language will also change over the course of 500 years

500 year old English is nothing like English today

Even plain text is no guarantee. This assumes that future people will read/understand/use latin characters. ASCII could very well be replaced in the medium-term future

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.

Indeed. As I commented separately I think the only practical solution is to use a simple medium (USB stick) and simple, standard file formats, then to copy from medium to medium, and to convert obsolete formats as time passes.

This requires descendants to keep at it over time, and it not really a "web site", but IMHO is the only way to keep the date accessible and useable over time.

Broken but not illegible. I feel like you're arguing that because the world may forget how to read some data 500 from now, storing the data is worthless. But that's just not true. Archeologists find meaning in writings they've never seen before all the time.