Image showing classes

I am dealing with an HTML document. The document consists of divs, and each div has a number of classes. One of these classes tends to set the “left” attribute of the div. I am writing Javascript code and want to identify which class is setting the left attribute.

For example, I might have <div class=”xa h3 y1d fs0 fc0″>. These classes all define some style property, but only xa (as shown in the image) is defining the left attribute.

How can I write code to identify which of these classes is setting the attribute?

I have tried using getComputedStyle, but to my knowledge there is no way to specify which class the attribute is coming from.