When using the web page app in the Designer the web page corresponding to the URL that is used sometimes does not show in the web page app.
Web browsers are not allowed to show what is called "mixed content". This means that they will not show content (parts of a web page) that is transported over an "insecure" protocol ie http when the main part of the web page is transported over a secure protocol ie https. This is a safety feature of web browsers and cannot be circumvented.
When the URL of the web page that you want to show starts with http and the Designer is opened using https then a "mixed content" situation is created. The Designer is normally opened over http to prevent the "mixed content" situation. However the Designer is opened over https when:
The simplest solution to this problem is to change the URL used in the web page app to use https (change "http" to "https").
A small minority of websites does not work when using https. This can be tested by entering the https-URL in a browser and checking that the site opens. If a website does not work using https it is not possible to use https in the URL. In that case and the Enforce playback over HTTPS setting is not set the problem lies in the browser.
To fix the problem in the browser you can do the following: Chrome
Firefox