From AWS documentation I can see that CPU and Memory properties are required in the AWS::ECS::TaskDefinition for Fargate but not in the ContainerDefinition within the resource if Fargate is used.
How does this exactly work? If I do not specify it in the ContainerDefinition it will use as much resources the Task have available? If there is only one container within the task... does it make any sense defining those values? If they are required, it seems pretty redundant and verbose to me.
When a container has no specified limits in a TaskDefinition, the container will use all the available resources for the task, which are mandatory for a Fargate task.
This means that there is no need to define them if there is only one container in the TaskDefinition. They can be specified though they are redundant (if equal to those of the task itself) or even harmful (if they are lower than the amount given to the task).
In case more than once container belongs to the same task, Fargate will distribute evenly the resources among all the containers. This may (or not) be a desired behaviour.