Earlier quoted context omitted.
AMP literally has a built in 1 second delay to render outside of Google, non AMP content, that is the "Nice comparative boost". It has nothing to do with "pre rendering" its built right into the CSS.
how is this done in CSS?
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).