I am a bit confused by that. I assume be meant <!DOCTYPE html> because he does not seem to have any any reason not to use it: it is what one of the sites he links to recommends, and it is what the other uses.
Browsers are depended on W3C recommendations. As Quirks mode is not a standard. It means you have correct DOCTYPE declaration like <!DOCTYPE html>, but just <html> will turn on quirks as this is not correct DOCTYPE declaration.
I really don't understand the question/issue, and I'm guessing others don't either -- You're saying without a preceding doctype declaration:
<html> <head> blah blah blah
Will turn on quirks mode, but
<!DOCTYPE html> <html> <head>
blah blah blah
Will not -- Correct so far?
If I am, and the answer to the question is not: "Use <!DOCTYPE html><html> rather than simply an opening <html> tag.", then please, ask the question again or give us some clue why someone who had access to a webpage to add code, etc. would use <html> rather than <!DOCTYPE html><html>, because it really makes no sense to me and if someone has enough access to add code, it certainly seems like they should have enough access to add a doctype declaration, so why on earth would they choose not to?