As described in this Let's Encrypt blog entry, certificates issued by Let's Encrypt will soon be signed solely by that organization's own root certificate, which is accepted by all modern browsers. There is one little catch, though: versions of Android prior to 7.1.1 (released in late 2016) do not recognize that certificate and will start throwing errors. "Currently, 66.2% of Android devices are running version 7.1 or above. The remaining 33.8% of Android devices will eventually start getting certificate errors when users visit sites that have a Let’s Encrypt certificate. In our communications with large integrators, we have found that this represents around 1-5% of traffic to their sites." There appears to be little to be done about this problem other than to encourage owners of older Android devices to install Firefox.