Let's Encrypt 宣佈他們的憑證被所有主流瀏覽器直接支援了，也就是都在各平台的 trusted store 內了：「Let's Encrypt Root Trusted By All Major Root Programs」，看起來這次加入的是 Microsoft 的產品：
As of the end of July 2018, the Let’s Encrypt root, ISRG Root X1, is directly trusted by Microsoft products. Our root is now trusted by all major root programs, including Microsoft, Google, Apple, Mozilla, Oracle, and Blackberry.
先前是靠 IdenTrust 的 cross signing 讓各瀏覽器信任，在「Chain of Trust」這邊有圖說明：
另外查了一下 cross signing 的資訊，以 Let's Encrypt Authority X3 這張的 cross signing 可以看到 cross sign 簽了五年，到 2021 年：(憑證可以從 letsencryptauthorityx3.pem.txt 這邊取得，用 openssl x509 -in letsencryptauthorityx3.pem.txt -text 看到)
Signature Algorithm: sha256WithRSAEncryption
Issuer: C = US, O = Internet Security Research Group, CN = ISRG Root X1
Validity
Not Before: Oct 6 15:43:55 2016 GMT
Not After : Oct 6 15:43:55 2021 GMT
Subject: C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
這樣約滿不知道會不會續簽...