When Javascript is disabled, then you won’t be able to open or close the menu, it will be just shown as you see it now, when you go to any page, and don’t touch it.
So you have two options, either to turn this off, and your menuitems, which should be closed will be closed before the menu shows up, but then some of your menuitems won’t be reachable when the javascript is disabled, or to make all of your menuitems opened before it loads, and if the javascript is enabled, it will close up what should be closed, and if the javascript is disabled, then every menuitem will be reachable.
I don’t think you should worry much about people, who are disabling javascript in their browsers, because it’s just a bad thing to do, almost every website is using javascript including yours, and probably it wouldn’t be perfect anyway, and those people are used to not seeing everything perfectly, I think they are rather suprised, when they found a website looking and working as it should be. Also it’s between 0 and 2% of all people, which is less, than people using really old or rare browsers, which aren’t supported by most websites either.
HTML5 mode means, that the menu will be around <nav></nav>
tags. A few years ago people thought, that it’s increasing their google ranks, that is why it was implemented, but nowadays it probably doesn’t matter.
The menu looks the same in mobile devices as in computers, it’s just a sidebar menu, so it won’t change.