var currentTextSize = 12;
var currentLineHeight = 18;

function textSize(dir) {
    if (dir == 'up') {
        if (currentTextSize <= 16) {
            currentTextSize += 2;
        }
    } else if (dir == 'down') {
        if (currentTextSize >= 14) {
            currentTextSize -= 2;
        }
    }
    currentLineHeight = currentTextSize + 4;
    document.getElementById('content').style.fontSize = currentTextSize + 'px';
    document.getElementById('content').style.lineHeight = currentLineHeight + 'px';
}