Why does the BUI fail to load a browser control in a workspace?

Environment:

Browser User Interface (BUI)

Resolution:

For the best experience using the browser control in BUI:

1. In a browser trying to render an HTTPS site inside an HTTP site or vice versa, modern browsers rendering of mixed content is treated as a security violation.

2. Any site which does not want to be embedded inside an iFrame will not be rendered. For example, if the response header containing X-FRAME-OPTIONS is set to SAMEORIGIN , the site will not be rendered.
MDN Web Docs Mozilla: X-Frame-Options

3. Any site trying to navigate a top level page will not be rendered by default due to the iFrame being loaded as a sandboxed attribute. For example, a BUI site navigating to a different site will not be allowed. If you have external content that needs to be loaded in the sandboxed iFrame in this way, such as SSO/External content, please review the answer here on setting up the CUSTOM_CFG_BUI_IFRAME_DOMAIN_LIST. 
Browser Agent UI (BUI) Module fails to load with 500 error

4. The Connect Desktop Integration JavaScript API is not supported in BUI workspace browser controls. Service Cloud customization elements used in object workspaces are best developed using the Agent Browser UI Extensibility Framework. For external links, in many cases, it is best to open a new tab or browser window to reference an external link, rather than loading it from within a browser control.