JavaScript get Styles

You are talking about what is known as Computed Style, check out these article on how to get it:

• Get Styles on QuirksMode
• Get Computed Style
• Get the rendered style of an element

From the last article, here is a function:

function getStyle(oElm, strCssRule){
    var strValue = "";
    if(document.defaultView && document.defaultView.getComputedStyle){
        strValue = document.defaultView.getComputedStyle(oElm, "").getPropertyValue(strCssRule);
    }
    else if(oElm.currentStyle){
        strCssRule = strCssRule.replace(/\-(\w)/g, function (strMatch, p1){
            return p1.toUpperCase();
        });
        strValue = oElm.currentStyle[strCssRule];
    }
    return strValue;
}

How to use it:

CSS:

/* Element CSS*/
div#container{
    font: 2em/2.25em Verdana, Geneva, Arial, Helvetica, sans-serif;
}

JS:

var elementFontSize = getStyle(document.getElementById("container"), "font-size");