I tried the new release of that css menu : but the main problem of this new, and responsive release (with some aiax script, though) it that by clicking on the hamburger icon all menu levels are shown. And If you have more than 50 entries it is a too long list.
To get a drop-down pure css reponsive menu, because of the limits of our current browsers, it is necessary use input and label tags (in a terrible: non-sematic way). This is the way of this attempt: no bad (no js, only css and that trick), but very hard to import in a previous menu.
Another attempt was following an example of collapsible responsive menu w3cschools: a code very simple and lovable, so to say, but here too: very difficult import in a previous, complex, menu.