I think about this a lot, and I prioritize site longevity and compatibility in my framework. To this end, I leverage the Lindy Effect, writing HTML which works with the last 25 years of browsers, starting with Mosaic and Netscape. You want to use only the most basic, least common denominator HTML markup, to improve its chances of not becoming "deprecated".
If you use JS and CSS, it must have abundant feature-checks, and ideally be optional. Your pages should certainly be usable without JS. You probably want to go with static HTML or only the most basic, again, lowest common denominator server-side dependencies, such as SSI.
You must make your site easily indexable, crawlable, and spiderable, so that it can be easily propagated to the Internet Archive and other archiving systems. Many sites I made in the past are long gone at their original URLs but remain accessible via Wayback Machine.
If you use JS and CSS, it must have abundant feature-checks, and ideally be optional. Your pages should certainly be usable without JS. You probably want to go with static HTML or only the most basic, again, lowest common denominator server-side dependencies, such as SSI.
You must make your site easily indexable, crawlable, and spiderable, so that it can be easily propagated to the Internet Archive and other archiving systems. Many sites I made in the past are long gone at their original URLs but remain accessible via Wayback Machine.
That's all I can think of for now.