2.2.42 [W3C-HTML51] Section 6.7.11 Unloading documents

C0058: The prompt does not show the returnValue

The specification states:

 ... Unloading documents
     ...
     When a user agent is to prompt to unload a document, it must run the following steps.
         1. ...
         ... ...
             The prompt shown by the user agent may include the string of the returnValue 
             attribute, ...

EdgeHTML Mode

The prompt does not show the returnValue.