How is an archived web site supposed to be a good resource on modern html and css development exactly?

The principles are the same: deliver content in an accesible way separating content from presentation, and if needed use javascript to improve usability, not to hijack basic site functionality.

Unfortunately he died in 2024, that's why the website is down. His last articles are available at Medium: https://deathshadow.medium.com/

These ones are a good starting point to develop accesible websites:

https://web.archive.org/web/20221117231315/https://cutcodedo... https://web.archive.org/web/20221117231314/https://cutcodedo...

We plateaued about 10 years ago right?