XSS can occur in GNOME Web (aka Epiphany) before 40.4 and 41.x before 41.1 via an about: page, as demonstrated by ephy-about:overview when a user visits an XSS payload page often enough to place that page on the Most Visited list.
XSS can occur in GNOME Web (aka Epiphany) before 40.4 and 41.x before 41.1 because a server's suggested_filename is used as the pdf_name value in PDF.js.
XSS can occur in GNOME Web (aka Epiphany) before 40.4 and 41.x before 41.1 when View Source mode or Reader mode is used, as demonstrated by a a page title.