Title is actually optional “if the document is an iframe srcdoc document or if title information is available from a higher-level protocol” (such as email). So the minimal valid HTML document is actually just <!doctype html>.

Declaring that it’s not an HTML file is a weird way to claim the smallest valid HTML file.

That has nothing to do with HTML. That's an HTTP header.

This thread has a surprising amount of not knowing the difference between HTML and HTTP. The original post is entirely about HTML and does not involve HTTP transmission at all.

HTML <meta> tags are a thing, to invoke browser behavior typically associated with HTTP headers. I'm not sure if that can get you there; I would guess that to visually output the meta tag would require the same * { display:block } shenanigan that the original post is about.