Markdown is thirteen years old. This is older than HTML5, which introduced '<article>', '<section>', '<header>', '<footer>', '<figure>', and '<aside>'; and it's older than lots of modern CSS features. I've got archives in a variety of markup languages going back several decades; I've found old Markup easier to work with than old HTML or SGML. YMMV, though.

If your documents are available on the open web, you can visit them on the Wayback Machine to make sure they're cached as HTML. This is in some ways more reliable, and some ways less, than trying to keep track of your own HTML transcodes over decades. It's generally a more reliable way of making sure they're available after you disappear.