JSFiddle - React, Tailwind, and code Playground

HTML

<span id="btn">click to toggle</span>
<div id="regform">
    <p>Hi. This is regform.</p>
</div>
<p>This is another element</p>

CSS

#regform {
    -webkit-transition:opacity 300ms;
    -moz-transition:opacity 300ms;
    -o-transition:opacity 300ms;
    -ms-transition:opacity 300ms;
}

JavaScript

var toggle = function(elm){
    var y = elm.style && elm.style.display ? elm.style.display : 'block';
    if (y == 'block'){
        elm.style.opacity='0';
        setTimeout( function(){ elm.style.display = 'none'; }, 300 );
    }
    else {
        elm.style.display='block';
        setTimeout( function(){ elm.style.opacity = '1'; }, 10 );
    }
}

document.getElementById('btn').onclick = function(){
    toggle(document.getElementById('regform'));
}