The ="" there is unnecessary; writing an attribute name only implies an empty value. If you’re going for shortest valid document at all, <!doctype html> is eno

The ="" there is unnecessary; writing an attribute name only implies an empty value. If you’re going for shortest valid document at all, <!doctype html> is enough, provided the document exists in a context that makes title optional (e.g. email or <iframe srcdoc>) and that the charset is declared out-of-band.