Example :

For example in the top window (Parent): (the_iframe.contentWindow.postMessage('data', 'domain');)

Option 1.

Use the drupal 8 native URL Field option.

Option 2. Module PathAuto

- Configure the path settings and Path pattern 

-> NOTE : Do not change the path

Tuto English : 

CSS Box Model : http://www.w3schools.com/css/css_boxmodel.asp

CSS display Property - http://www.w3schools.com/cssref/pr_class_display.asp

Use drupal 8 httpClient to read URL.

// Drupal httpClient.

$http = \Drupal::httpClient();

$url = 'http://example.com/';

Example:

$options = array();

$result = $http->request('get', $url, $options);

Example 1 : With DOMDocument: (Fast (0.0002 sec) but can have php warning)