var textSize = 13;
var lineHeight = textSize + 7;


function plus() {
        if (textSize <= 17) 
            textSize += 2;
    lineHeight = textSize + 7;
    document.getElementById('art').style.fontSize = textSize + 'px';
    document.getElementById('art').style.lineHeight = lineHeight + 'px';
	return false;
}		

function minus() {
    if (textSize >= 13) 
         textSize -= 2;
    lineHeight = textSize + 7;
    document.getElementById('art').style.fontSize = textSize + 'px';
    document.getElementById('art').style.lineHeight = lineHeight + 'px';
	return false;
}