Often customer requirement is to display some external web page inside POS. If you try to embed iframe control inside Modern POS (MPOS), the following error will appear:
Unable to navigate to: ‘https://website.net’. An iframe attempted to navigate to a URI that is not included in the ApplicationContentUriRules for this app. Use a x-ms-webview element to view the URI instead, or add the URI to the ApplicationContentUriRules section of the package manifest so that the iframe can navigate to it. (In Visual Studio, add this URI to the Content URIs tab of the Manifest Designer.)
As error says, iframe tag should be replaced with x-ms-webview tag in HTML.
What if we use both CPOS and MPOS?
Add both iframe and x-ms-webview elements to HTML and enable/disable them depending on application type using Knockout IF condition.