Approximate IE6/IE7 quirks detection logic:

function is_nonquirks() {
    if (contains("DTD HTML 4") && contains("http://")) {
        return true;
    } else if (contains("DTD HTML 4.")) {
        if (contains(" Transitional//") || contains(" Frameset//"))
            return false;
        return true;
    } else if (contains("DTD XHTML")) {
        return true;
    } else if (
           contains("DOCTYPE NETSC")
        || contains("DTD HTML//")
        || contains("DTD HTML EXP")
        || contains("DTD W3 HTML//")
        || contains(" HTML 2")
        || contains(" HTML 3")
        || contains(" HTML 4")
        || contains(" HTML plus")
    ) {
        return false;
    } else {
        return true;
    }
}

where 'contains' is case-insensitive, and matches everything inside the <!doctype ...> (including that "doctype ").