Grazie per il consiglio, ho letto tutta la guida, ho provato a fare così ora, ma il div che all'interno del div principale vorrei mettere non lo prende in considerazione (mi riferisco all'innerHTML verso la fine dello script)

codice:
var oTooltip = new (function() {
	var nOverX, nOverY, nLeftPos, nTopPos, div, bOff = true;
	this.follow = function (oMsEvnt1) {
		if (bOff) { return; }
		if (!oMsEvnt1) {  oMsEvnt1 = window.event; }
		var nMoveX =  oMsEvnt1.clientX, nMoveY =  oMsEvnt1.clientY;
		nLeftPos += nMoveX - nOverX; nTopPos += nMoveY - nOverY;
		div.style.left = nLeftPos + "px";
		div.style.top = nTopPos + "px";
		nOverX = nMoveX; nOverY = nMoveY;
	};
	this.remove = function () {
		if (bOff) { return; }
		bOff = true; document.body.removeChild(div);
	};
	this.append = function (oMsEvnt2, sTxtContent) {
		if (!oMsEvnt2) { oMsEvnt2 = window.event; }
		div.innerHTML = sTxtContent;
		if (bOff) { document.body.appendChild(div); bOff = false; }
		var nScrollX = document.documentElement.scrollLeft || document.body.scrollLeft, nScrollY = document.documentElement.scrollTop || document.body.scrollTop, nWidth = div.offsetWidth, nHeight = div.offsetHeight;
		nOverX = oMsEvnt2.clientX; nOverY = oMsEvnt2.clientY;
		nLeftPos = document.body.offsetWidth - nOverX - nScrollX > nWidth ? nOverX + nScrollX + 10 : document.body.offsetWidth - nWidth + 16;
		nTopPos = nOverY - nHeight > 6 ? nOverY + nScrollY - nHeight - 7 : nOverY + nScrollY + 20;
		div.style.left = nLeftPos + "px";
		div.style.top = nTopPos + "px";
	};
	this.init = function() {
		div = document.createElement('div');
		div.style.position = "absolute";
		div.innerHTML = "<div style='position: absolute; width: 50; height: 20; background: #ff0000'></div>";
	};
})();