I think there is confusion about the term "rendering" here. The term "rendering" is used (a) to describe the actual translation of textual-instructive HTML/CS

I think there is confusion about the term "rendering" here. The term "rendering" is used (a) to describe the actual translation of textual-instructive HTML/CSS/Javascript into a graphical representation within a browser window and (b) to describe stuff being loaded into a page via Javascript, most prominently ads, and more oriented towards load time ("How long until the page has finished loading" = "rendering time"). With CSS, you can define pixel-perfect areas for stuff to be rendered into, which makes the browser not having to decide on the size of the area after it has loaded further material from third-party sources. Doing so reduces "screen flickering", which is generally annoying (and the major reason why I block ads with a vengeance). This can be described as "pre-rendering" when you understand rendering as (a).