Retrieving fontsize

Hi
I'm just trying to incorporate a dynamic font sizing function into an application. On this test, I would have say 4 lines of various CSS-styled lines. Two buttons would be present to decrease and increase the size... the function is below, but isn't working - I can't seem to retrieve the actual font-size:

Anybody know why this is??

Code:

---

var tags = new Array( 'div','td','tr','p','b','table','a');
var fa;


function fontSize(amt) {
    if (!document.getElementById)
        return;
        getBody = document.getElementsByTagName('body')[0];

    for (i = 0 ; i < tags.length ; i++ ) {
        getallTags = getBody.getElementsByTagName(tags[i]);
        for (k = 0 ; k < getallTags.length ; k++) {
            fa=getallTags[k].style.fontSize;
            fa+=amt;
            getallTags[k].style.fontSize=fa+"px";      }

    }
}

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