JSFiddle - React, Tailwind, and code Playground

CSS

body,html{
 height:100%;
 margin:0;
 padding:0;
 background-color:#666;
}
*{
 box-sizing:border-box;
}
.clc{
 width:256px;
 height:320px;
 clear:both;
}
.clc>*{
 border:4px solid #666;
 border-radius:4px;
 font:normal normal bold 20px/56px Arial,Helvetica,sans-serif;
 border-radius:10px;
}
.clc>input{
 width:100%;
 height:20%;
 text-align:right;
 padding:0 20px;
}
.clc>div{
 width:25%;
 height:20%;
 background-color:#5a5a5a;
 color:#fff;
 text-align:center;
 float:left;
}
.clc>div:nth-child(4n+1){
 background-color:#f36573;
}
.clc>div:nth-last-child(1){
 width:100%;
 background-color:#ffb343;
}

JavaScript

function calc(e){
 var a=e.target.innerText,b=this.firstChild;
 b.value=a=='='?eval(b.value):a=='C'?'':b.value+a;
}
gui=document.createElement('div');
gui.className='clc';
gui.innerHTML='<input><div>'+('789+456-123*C0./='.split('').join('</div><div>'))+'</div>';
gui.addEventListener('click',calc,false);
document.body.appendChild(gui);