JSFiddle - React, Tailwind, and code Playground

HTML

<button class="btn" onclick="myfunc()" id="btn0"> Cool button</button>

<div id="test">
  Hello
</div>

CSS

.btn{
  border: none;
  color: white;
  width: 100%;
  padding: 5px 5px 5px 5px;
  height: 50px;
  cursor: pointer;
  font-size: 12px;
  background-color: #343a40;
}

.btn:active .btn.active{
  background-color:green;
  outline: none;
}

.btn:focus{
  background-color:green;
  outline: none;
}

#test{
  background-color: green;
  width: 600px;
  height: 400px;
}

JavaScript

var myfunc = function(){
	document.getElementById("test").style.backgroundColor="red";
};

document.getElementById("btn0").click();