function fontSize(p) {
	var difference = 0.2;

	if (p != "+") {
		difference *= -1;
	}

	var elems = document.getElementsByTagName("div");

	for(var i=0; i < elems.length; i++){
		if(elems[i].getAttribute("srtReadable")){
			if(elems[i].style.fontSize == ""){
				elems[i].style.fontSize = (parseFloat(elems[i].getAttribute("srtReadable")) + difference) + "em";
			}
			else{
				var currentSize = parseFloat(elems[i].style.fontSize);

				if((currentSize >= parseFloat(elems[i].getAttribute("srtReadable")) && difference < 0) || (currentSize <= (3 * parseFloat(elems[i].getAttribute("srtReadable"))) && difference > 0)){
					elems[i].style.fontSize = (currentSize + difference) + "em";
				}
			}
			
			//change font elements
			fontElems = elems[i].getElementsByTagName("font");

			for(var j=0; j < fontElems.length; j++){
				fontElems[j].size = parseInt(fontElems[j].size) + ((p != "+") ? -1 : 1);
			}
		}
	}
}