After a user has logged in to IdP and the IdP sends the user back to the requesting web service a white page without any information is shown under a brief period. Some web services take a long time to log in the user and the services do not present their own "please wait page". The white page is a self posting web form with special handling for browsers without script support. It has been possible to adapt this white page with a standard message but until Shibboleth IdP 2.4.0 this was not officially supported by the Shibboleth Consortium and is documented on the Shibboleth Wiki page AddPostPageHeadBodyContent.
The simple solution
In Shibboleth IdP 3.x you can create a file add-html-body-content.vm with the selected message and put in the Shibboleth directory views/templates. The files that invoke the template file is script files with support for no script so therefore you should use scripting to display the message. After the file is put in to the directory run the Shibboleth install script to get the template file into the running code.
Example of add-html-body-content.vm:
This behavior is the same as in Shibboleth IdP 2.4. For more information please see AddPostPageHeadBodyContent on the Shibboleth Wiki.