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 (CVE-2021-45085).
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 (CVE-2021-45086).
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 (CVE-2021-45087).
XSS can occur in GNOME Web (aka Epiphany) before 40.4 and 41.x before 41.1 via an error page (CVE-2021-45088).