Set iframe height and width to no more than content using CSS

I have an iframe with content which is dynamic in height and width.

<iframe id="myIframe" src="../xxx/xxx" scrolling="no" frameBorder="0"></iframe>

I am able to change the height and width using JavaScript so that my iframe is no bigger than it's content.

var iframe = document.getElementById("myIframe");
iframe.height = iframe.contentWindow.document.body.scrollHeight + 'px';
iframe.width = iframe.contentWindow.document.body.scrollWidth + 'px';

How can I do this using only CSS? Thanks to anyone who can provide guidance.