Small sites, long lifespans
The websites that survive for decades tend to be the least ambitious ones: a few static files, no database and nothing that needs constant updating.
Fewer moving parts
Every dependency is a promise someone else has to keep. A site made of plain files only depends on a web server, which is about as stable as software gets.
Plain formats
HTML, CSS and plain text will still open in thirty years. Proprietary formats and build tools often will not.
Write for the long term
Give pages stable addresses, avoid clever scripts and keep the content easy to move. Those habits cost almost nothing and pay off for a very long time.