> <!doctype html> is what you want for consistent rendering. Or <!DOCTYPE HTML> if you prefer writing markup like it’s 1998. Or even <!doCTypE HTml> if you eschew all societal norms. It’s case-insensitive so they’ll all work.
And <!DOCTYPE html> if you want polyglot (X)HTML.
I tend to lower-case all my HTML because it has less entropy and therefore can be compressed more effectively.
But in case of modern compression algorithms, some of them come with a pre-defined dictionary for websites. These usually contain the common stuff like <!DOCTYPE html> in its most used form. So doing it like everybody else might even make the compression even more effective.
We need HTML Sophisticated - <!Dr. Type, HtML, PhD>