Previously Project for the web only supported finish to start (FS) dependencies between tasks. Rolling out now is the ability to create different task dependencies so you are no longer limited to FS, you can now also use finish to finish (FF), start to start (SS) or start to finish (SF).
These can be set from the Task details pane on all views such as below:
But also from the Depends on and Dependents columns on the Grid view:
The timeline only supports rendering the different dependency types:
Creating task dependencies on the timeline view using drag and drop creates finish to start dependencies.
A much needed addition to Project for the web!
Modern Work Management - Project and the Power Platform blog 