HTML best practices Depends. How far down the rabbit hole do you want to go ?
Just need something to look good : use a frontend framework like foundation or bootstrap, won't learn much about CSS. Short list of Front End Frameworks
Surface level : work general to specific. Comment the sections of the CSS. Use a reset or normalizer. Work on patterns, use classes over IDs for reuse. CSS Resets - includes normalize.css Don't have links for the other parts. see below
Little bit deeper : Look into BEM or OOCSS. Those stand for Block Element Modifier and Object Oriented CSS. Two popular methodologies for CSS. Also SMACSS.