function ChangeFontSize(element,offset) {
    if (element.css('font-size')) {
        var s = parseInt(element.css('font-size').replace('px', ''));
    } else {
        var s = 12;
    }
    if ((offset < 0 && s <= minFont) || (offset > 0 && s >= maxFont)) 
    {
        return; 
    }
    s += offset;
    if (s >= minFont || s <= maxFont) {
        element.css('font-size', s + 'px');
    }
}

var minFont = 8;
var maxFont = 24;
function increaseFontSize(elements) {
    $(elements).each(function() {
        ChangeFontSize($(this), 1);
    });
}
function decreaseFontSize(elements) {
    $(elements).each(function() {
        ChangeFontSize($(this), -1);
    });
}