news.com.au's markup is still based on HTML patterns I wrote c2008. At one time the entire news ltd metro stable ran on that markup standard (precursor to what I'd now call a UI library). Lots of details have changed but the core page sections, group/item/block, etc are unchanged. It's pretty cool to see that a site built to support IE6 with float layouts is still using the same HTML, but now running CSS Grid.
Griffith University's phone book pages still use the vcard microformat HTML I cooked up with a coworker c2006. The design is still basically the same too (barring a broken image ;)), I made the listing look the standard business cards at the time.
The very oldest code would be web pages I built back in 1996 - since they are flat/static files they still work. That's a testament to the web being backwards compatible and not a testament to my code ;)