getBoundingClientRect vs getComputedStyle

HTML

<div id="outer">
<div id="inner">

</div>
</div>
<div id="output">

</div>

CSS

#outer{
  width:18px;
}
#inner{
  width:90%;
}

JavaScript

var inner = document.getElementById("inner");
var output = document.getElementById("output");

var boundingRectWidth = inner.getBoundingClientRect().width;
var computedWidth = parseFloat(window.getComputedStyle(inner).width);

output.innerHTML="getBoundingClientRect: "+boundingRectWidth+"<p>computedWidth : "+computedWidth;