"Securing your self-hosted website with Let’s Encrypt, part 2: HTTPS and certificate authorities" has moved
You can now find it at /posts/2016/securing-your-self-hosted-website-with-lets-encrypt-part-2-https-and-certificate-authorities/. If you don't do anything, you'll be redirected automatically (if your browser supports that).
Or you can update your bookmarks, if you still use such a thing!