//Specify affected tags. Add or remove from list:
var tgs = new Array('div', 'td', 'tr');

//Specify spectrum of different font sizes:
var szs = new Array('xx-small', 'x-small', 'small', 'medium', 'large', 'x-large', 'xx-large');
var startSz = 2;

function ts(trgt, inc) {
    if (!document.getElementById) return
    var d = document,
    cEl = null,
    sz = startSz,
    i,
    j,
    cTags;

    sz += inc;
    if (sz < 0) sz = 0;
    if (sz > 6) sz = 6;
    startSz = sz;

    if (! (cEl = d.getElementById(trgt))) cEl = d.getElementsByTagName(trgt)[0];

    cEl.style.fontSize = szs[sz];

    for (i = 0; i < tgs.length; i++) {
        cTags = cEl.getElementsByTagName(tgs[i]);
        for (j = 0; j < cTags.length; j++) cTags[j].style.fontSize = szs[sz];
    }
}