In a formal sense, all attributes are optional. However, there are some that must be specified for an element to accomplish its purpose. Use the documentation as a guide. For truly optional attributes, it mentions a default value or states what happens in the absence of a specification.

Except for some attributes of the root <manifest> element, all attribute names begin with an android: prefix — for example, android:alwaysRetainTaskState. Because the prefix is universal, the documentation generally omits it when referring to attributes by name.

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