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CalDAV and CardDAV are available in Vivaldi’s Webmail.
When you log in with an existing account, it will ask you to import your calendars to CalDAV so you can synchronize them with your phone and other Calendar clients. Agree to it.
Once your calendars are imported. Look for the calendar link to share to your client here:
Then use the following details to add it to your client
To synchronize your calendar, use this url to connect it to Vivaldi servers: https://calendar.vivaldi.net/calendars
To synchronize your contacts, use this url to connect it to Vivaldi servers: https://calendar.vivaldi.net/addressbooks
Then add your credentials from your Vivaldi Account ([email protected] + your password).