How do browsers read and interpret CSS?

Question

Two part question:

1. Do browsers have a built-in CSS interpreter like they do for JavaScript?
2. When exactly does a browser read the CSS and when does it

Answer 1

Yes, browsers have a CSS interpreter built in. The reason you don't "wait until window.onload" is because while Javascript is a Turing-complete imperative programming language, CSS is simply a set of styling rules that the browser applies to matching elements it encounters.