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I tested this out and found that some websites would show up while others will not. For example, using https://www.wikipedia.org/ in a dashboard web page object will display when published while https://www.google.com/ will not.
For security reasons some websites chose to not allow loading of the page in an iframe tag. You can see some available workarounds here:
Web Page Object Linked to Web-Based URL Blank After Publishing | Tableau Software