How to get an HTML element's style values in JavaScript?

I am looking for a way to retrieve the style from an element that has a style set upon it by the style tag.

<style> 
#box {width: 100px;}
</style>

In the body

<div id="box"></div>

I'm looking for straight javascript without the use of libraries.

I tried the following, but keep receiving blanks:

alert (document.getElementById("box").style.width);  
alert (document.getElementById("box").style.getPropertyValue("width"));

I noticed that I'm only able to use the above if I have set the style using javascript, but unable to with the style tags.