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.
XSS can occur in GNOME Web (aka Epiphany) before 40.4 and 41.x before 41.1 via an error page.
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 suggestedfilename is used as the pdfname value in PDF.js.