That's why it's nice of Let's Encrypt to publish their current and backup intermediate CAs (X3 and X4) https://letsencrypt.org/certificates/. That's what I pin for my sites.
[1]: https://community.letsencrypt.org/t/official-hpkp-support-fr...
In support of being cautious about predicting what intermediate will be used for issuance at a given time, there was also once a Let's Encrypt Authority X2 (also browser-trusted by virtue of being signed by IdenTrust's root), but issuance skipped directly from the X1 to the X3 intermediate in March 2016.
https://letsencrypt.org/certs/lets-encrypt-x2-cross-signed.p...