Understand more about Office.js, it’s add-in runtime environment and Internet Explorer protected mode!!

In addition to the earlier blog post, you can refer the following:

- Microsoft Office addins using the Apps for Office platform are designed to run in isolation, using a low rights sandbox.
- Several features, such as the task pane view, use web views provided using Internet Explorer APIs. 
- To ensure these components execute HTML script in low rights mode, Office explicitly requires the browser object to run in Protected Mode. 
- Currently, IE does not support this from the API unless Protected Mode is enabled for Restricted Sites zone in IE itself.  This is typically not a problem since this is the default setting, and rarely will users disable it. However, if disabled (by user action or by GPO), these Office addin types may report an error when starting and refuse to load. It is an expected failure under that condition, and is blocked for security reasons.
- So while using Windows desktops, Protect Mode in Internet Explorer must be enabled for the Restricted Site Zone. This is typically enabled by default. If it is disabled, an error will occur when you try to launch an add-in.

For more info, you can refer

- https://msdn.microsoft.com/en-us/library/bb250462(v=vs.85).aspx to know more about “Understanding and Working in Protected Mode Internet Explorer”
- https://dev.office.com/docs/add-ins/develop/privacy-and-security to know more about “Privacy and security for Office Add-ins”

Hope this helps.