And to spell it out:

- Stylesheet encoded as base64 in the Link header;

- Browsers always implicitly have at least the html and body tags;

- CSS cannot create new elements, but it does get 2 free pseudo-elements per actual element, ::before and ::after;

- CSS can set textual content for pseudo-elements;

So, it sets content to a pseudo element of an implicitly created tag, that's why the page is so minimal.

(Well, it could be up to 4 times as complicated I think, by using the other 3 pseudo-elements. Or cheat by using a big SVG as a background-image with more complex contents, but then you start running into header size limits and whatnot)