[tudorw]

You could start by reading the specifications to get up to speed on the scope of contemporary HTML, not quite sure where you are coming from technically so if it sounds condescending just fill me in with where you stand now ;)

https://www.w3.org/standards/techs/html#w3c_all

[steindavidb]

Going straight to the spec is really great for HTML/CSS. Also worth looking at the CSS snapshot from w3 https://www.w3.org/TR/css-2017/