It looks like it is not possible to increase/decrease indent level on PowePoint Web App using the latest version of Google Chrome and Safari.
Buttons are greyed and tab doesn't work.
Do you have the same problem?
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Please try the same operation with a recent version of Internet Explorer.
According to the Browser Support sheet, Internet Explorer is better validated to support everything SharePoint has to offer:
Supported Internet Explorer versions:
The product group makes every effort to validate that SharePoint functionality works correctly with released versions of Internet Explorer. Customers who want a more deeply validated browser interaction experience should strongly consider Internet Explorer.
Internet Explorer 11, Internet Explorer 10, Internet Explorer 9, Internet Explorer 8