var textSize = 12;
var lineHeight = textSize + 5;

function up() {
        if (textSize <= 17) 
            textSize += 2;
    lineHeight = textSize + 5;
    document.getElementById('content').style.fontSize = textSize + 'px';
    document.getElementById('content').style.lineHeight = lineHeight + 'px';
	return false;
}		

function down() {
    if (textSize >= 11) 
         textSize -= 2;
    lineHeight = textSize + 5;
    document.getElementById('content').style.fontSize = textSize + 'px';
    document.getElementById('content').style.lineHeight = lineHeight + 'px';
	return false;
}