Attributes were chosen as the default so there's no question which transition is active. **There can be only 1 at a time.** With classes, for example, what happens when multiple "transition in" classes are applied to an element? Transition.css chooses to default with a state machine approach so things like a classname collision doesn't need solved. See the [custom](#custom) section below for ways to use classes and/or the shape custom properties so transition.css can fit into your development environment. The built in attribute based approach is very easy to hack, customize and escape.
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