function getElementsByClass(searchClass,node,tag) {
	var classElements = new Array();
	if ( node == null )
		node = document;
	if ( tag == null )
		tag = '*';
	var els = node.getElementsByTagName(tag);
	var elsLen = els.length;
	var pattern = new RegExp("(^|\\s)"+searchClass+"(\\s|$)");
	for (i = 0, j = 0; i < elsLen; i++) {
		if ( pattern.test(els[i].className) ) {
			classElements[j] = els[i];
			j++;
		}
	}
	return classElements;
}

var cms=getElementsByClass('cms',null,null);

cms[0].innerHTML='<div id="wstawka" style="width:980px; margin-left: -10px; position:relative; overflow: hidden"><iframe scrolling="no" width="980" height="800" frameborder="0" src="http://www.nexto.pl/promocja-dnia/?'+document.domain+'">Przegladarka nie obsluguje tej funkcji</iframe></div>';