Rewrite jQuery easing easeInExpo function into plain javascript & css

So I have this great little nugget of code that I am trying to rewrite into plain JavaScript and CSS without jQuery.

jQuery.extend(jQuery.easing,{
    easeInExpo: function (x, t, b, c, d) {
        return (t==0) ? b : c * Math.pow(2, 10 * (t/d - 1)) + b;
    }
}); 

var nset = false;
$('button#hmenu').click(function(){
    if(!nset){
        $('ul#nav').delay(35).slideDown(300,'easeInExpo');
        $('button#hmenu').addClass('active');
        nset = true;
    } else {
        $('ul#nav').slideUp(550);
        nset = false;
        $('button#hmenu').removeClass('active');
    }
})  

I'm looking at some CSS transitions that use easing but just wondering what the options are? In my code I have a slideDown and up easing functions. This is used in production for a mobile menu nav.

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