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Re:HTTPS on Stack Overflow: The End of a Long Road by tomschlick on Monday May 22, 2017 @12:05PM (#14394679)
Probably their agreements with their partner CAs. Given that those partners sell wildcards themselves...
Re:HTTPS on Stack Overflow: The End of a Long Road by prdonahue on Monday May 22, 2017 @04:59PM (#14396898)
Their "Let’s Encrypt Authority X3" intermediate is signed by their own root (ISRG Root X1). See https://letsencrypt.org/certificates/.