How to edit an external iframe?

I have read that you can change the content of an iframe external% by jQuery in the following way:

<script>
    $(document).ready(function () {
        $('#el_iframe').load(function () {
            $(this).contents().find("p").css({'background-color':'red','font-weight':'bolder','color':'white'});
        });
    });
</script>

Is it real? Are there other means to alter an iframe external%?