How Does Let’s Encrypt Gain Your Browser’s Trust?

This blog post describes how Let’s Encrypt achieves browser trust by using intermediate certificates signed by IdenTrust’s DST Root CA X3, explains the certificate chain and differences in operating system and browser certificate stores (with a note about Firefox bundling its own store and eventual inclusion of ISRG Root X1), clarifies the role of Let's Encrypt Authority X3/X4 for compatibility, and offers to provide a practical walkthrough for integrating Let’s Encrypt into an Nginx setup with zero downtime.