Fastmail's calendar is built on the Cyrus IMAP server's CalDAV support, which does all the open-standards. It also has the upcoming JMAP Calendaring support and JSCalendar (soon to be an RFC) format.

There's a couple of other good CalDAV servers out there too, though of course as Fastmail CEO I can't really advertise anybody else :p

Fastmail supports CalDAV (as does iCloud).

I use mailbox.org that has CardDAV and CalDAV in addition to email. I use DAVx⁵ for syncing to my Android phone. Mailbox.org's web UI is clunky but I rarely use it for anything other than extending the subscription. The calendar/contact sync and email have worked well.