Frontend & browser·Rendering and the browser pipeline
the page is blank until the stylesheet finishes downloading, even though the HTML arrived immediately.
CSSOM
Also calledCSS object model, style tree
The parsed representation of every stylesheet, which the browser must have in full before it can compute any element's styles. Unlike HTML, CSS cannot be applied incrementally: a rule at the bottom of the last stylesheet can override the first, so the browser blocks rendering until it has all of them. That is the whole reason stylesheets are render-blocking and scripts are not by default.