Teams preview won't open in Safari
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.
Changing this setting may impact your experience on other websites.
Once Safari is fully supported for Teams, you will no longer need this workaround.
When you're done using Teams in Safari, go to Preferences > Privacy and check the Prevent cross-site tracking setting again.
Read more about Intelligent Tracking Prevention here.