What are the requirements for CSS and JS in order to pass validation without hindrance, how to properly adapt the code for Yandex and Google?

Question

css

Everyone knows that the autoprefixer places additional prefixes for other browsers, but the final code does not pass validation. It's the same with babel. Which ultimately affects rankings. How to properly adapt the code for Yandex and Google. With Google, in principle, it is somewhat understandable, but with Yandex - like a hedgehog in a fog. Are there any requirements for the code, what to use ES6, ES5, ES8 in JS, the same about CSS, what you can forget about, and also what is better not to use for now. Are there any limits? At the W3C every time it's crazy to climb.