A browser-based, well. Browser. In this case, the OG 1990 version, called WorldWideWeb.
Since it’s not necessarily obvious by modern standards, this is a fully working web browser, albeit one that only supports the version of HTML that existed in, literally, 1990. But it will still browse to any modern website (Document › Open from full document reference), and it’s definitely… interesting to see what works and what doesn’t.1