do you own the external site? if so you can adjust CORS settings on both servers to allow cross domain scripts and css.
If you want to embed a full external website the easiest way is to use an iframe
<iframe id="myIframe" src=""></iframe>
extended answer from your comment. You can test the source via xhr and inject the iframe src if it gives a successful response. It's not 100% but probably the closest you'll get without compromising security.
// jQuery
$.ajax({
url: 'https://test.com',
dataType: 'JSONP',
type: 'GET',
async: false,
crossDomain: true,
success: function () { },
failure: function () { },
complete: function (data) {
if (data.readyState == '4' && data.status == '200') {
console.warn({ Status: 'SUCCESS' })
$('#myIframe').attr('src', 'https://test.com');
}
else {
console.warn({ Status: 'FAIL' })
}
}
});