Search Tag in Iframe from out of Iframe

this is Just example


document.getElementById('main_iframe').onload = function(){

var frameHtml = document.getElementById("main_iframe").contentWindow.document.body.innerHTML;
if(frameHtml.indexOf("loginForm") != -1){
location.href = '/neo/m4/home/login.mvc';
}else{

}

};


<iframe src="" id="main_iframe" width="100%" height="100%" marginheight="0" marginwidth="0" frameborder="0" scrolling="no"></iframe>