Safari 11.0 uses Intelligent Tracking Prevention to block third-party tracking cookies. Because of the way Intelligent Tracking Prevention categorizes cookies, login.microsoftonline.com gets lumped into this category (even though login.microsoftonline.com is not a tracking domain), and Microsoft Teams is prevented from opening.
If you want to use Teams in Safari while Safari browser support is in preview, go to Preferences > Privacy and uncheck the Prevent cross-site tracking setting. Then, close your browser and navigate back to teams.microsoft.com in Safari.
