This reminds me of something that was posted once, that I haven't been able to find again: A demo where a page returned content, but if you viewed the source it was 0 bytes.
If I remember right, it took advantage of a feature that let you reference CSS through the HTTP headers (so it wasn't in the HTML), then the CSS added things to implicit required elements the browser would add to the page.
Aha, I don't think it was that post (feels too recent and I don't remember the main post), but the much simpler demo I remember is linked in the comments there: https://css-tricks.com/using-css-without-html/
If I remember right, it took advantage of a feature that let you reference CSS through the HTTP headers (so it wasn't in the HTML), then the CSS added things to implicit required elements the browser would add to the page.