XAML Browser Applications
XAML Browser Applications (XBAP, pronounced "ex-bap") are Windows Presentation Foundation (.xbap) applications that were intended to run inside a web browser such as Firefox or Internet Explorer through the NPAPI interface. Due to NPAPI being phased out in recent years, and due to a lack of support, there are currently no browsers that support XBAP applications.
Source: Wikipedia — XAML Browser Applications (CC BY-SA 4.0)