Critical rendering path

The fixed sequence a browser runs to turn markup and styles into pixels: parse HTML into the DOM, parse CSS into the CSSOM, combine them into a render tree, lay it out, paint it, composite it. Nothing paints until the whole chain has run once, which is why a single stylesheet in the head delays the entire page. Optimising a page for first paint is almost always the work of removing something from this chain rather than making a step faster.