https://www.bleepingcomputer.com/news/security/lets-encrypt-is-now-officially-trusted-by-all-major-root-certificates/

Let's Encrypt announced yesterday that they are now directly trusted by all major root certificates including those from Microsoft, Google, Apple, Mozilla, Oracle, and Blackberry.  With this announcement, Let's Encrypt is now directly trusted by all major browsers and operating systems.

While Let's Encrypt has already been trusted by almost all browsers, it was done so through intermediate certificate that were cross-signed by IdenTrust. As IdenTrust was directly trusted by all major browser vendors and operating systems, it also allowed Let's Encrypt to be trusted as well.