`<!doctype html>` can be minified into `<!doctypehtml>`.

It’s, strictly speaking, invalid HTML, but it still successfully triggers standards mode.

See https://GitHub.com/kangax/html-minifier/pull/970 / https://HTML.spec.WHATWG.org/multipage/parsing.html#parse-er...

(I too use that trick on https://FreeSolitaire.win)