W3School:
https://www.w3schools.com/html/
https://www.w3schools.com/css/

Guidebook:
https://htmlandcssguidebook.com/

MDN Docs:
HTML - https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Structuring_content
CSS - https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Styling_basics

Neocities:
https://scripted.neocities.org