解决 ie6下 div层遮住select下拉框的问题

function obj_init(objdiv) {
var w = objdiv.offsetWidth;
var h = objdiv.offsetHeight;
var ifrm = document.createElement('iframe');
//ifrm.src = 'javascript:false';
//ifrm.style.cssText = 'position:absolute; visibility:inherit; top:0px; left:0px; width:' + w + 'px; height:' + h + 'px; z-index:-1; filter: /'progid:DXImageTransform.Microsoft.Alpha(style=0,opacity=0)/'';
ifrm.style.cssText = 'position:absolute; visibility:inherit; top:0px; left:0px; width:' + w + 'px; height:' + h + 'px; z-index:-1; ';
objdiv.appendChild(ifrm);
}