This policy setting allows you to configure the "Use Cascading Style Sheets (CSS) for appearance of messages" option found ...

This policy setting allows you to configure the "Use Cascading Style Sheets (CSS) for appearance of messages" option found under Microsoft Outlook's File tab | Outlook Options | Mail | Message format.

If you enable this policy setting, the option is checked.

If you disable or do not configure this policy setting, the option is not checked.