The doctype is a mode switch for the browser, not a label describing your page.

Browsers keep two rendering modes: a strict, modern one and a legacy one that copies old browsers' quirks. The first line of the file is the browser's cue for which to use. It says nothing about letters, language or titles; those each have their own home (a meta charset tag, the lang attribute, the title element).

Read it once, then a question tests whether the trick stuck.