HTMLIFrameElement.contentWindow

The contentWindow property returns the Window object of an <iframe> element. You can use this Window object to access the iframe's document and its internal DOM. This attribute is read-only, but its properties can be manipulated like the global Window object.

Example of contentWindow

var iframe = document.getElementsByTagName("iframe")[0];
var x = iframe.contentWindow;
if (x.document) {
    x = iframe.document; 
    // to ensure compability
} 
x.body.style.backgroundColor = "blue";
// this would turn the iframe blue.

Browser compatibility