JSFiddle - React, Tailwind, and code Playground

HTML

<div id="hold">
    <div id="hold-left">content</div>
    <div id="hold-right">content</div>
</div>

<select id="zoomLevelSelector">
<option value="1">normal</option>
<option value="2">large</option>
<option value="3">very large</option>
</select>

CSS

#hold{
	position:relative;
}

#hold-left{
	position:absolute;
	left:0;
	width:100px;
	height:100px;
  background:red;
}

#hold-right{
	position:absolute;
	right:0;
	width:100px;
	height:200px;
  background:yellow;
}

#zoomLevelSelector{
  position:absolute;
  top:0;
  left:0;
  z-index:0;
}

JavaScript

var selector = document.getElementById('zoomLevelSelector');
selector.addEventListener('change', function(){
	var right = document.getElementById('hold-right');
  right.style.width = 100 * this.options[this.selectedIndex].value + 'px';
});