Best resource for proper HTML+CSS development is Jason Knight AKA deathshadow's CUTCODEDOWN (cutcodedown.com at Web archive) and his articles at Medium and CodePen (user Jason Knight).
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...