> As in do you really want to test that a class name is applied to assert that the component rendered with a certain style or visual look?

I think the author doesn't do a good job in explaining what questions such tests are helping to answer. By checking the class name of an element in a component, you are not testing the visual appearance of the component, but its behavior. As in, given a certain set of inputs, the component is expected to produce a certain output. In the author's example, the difference in component's behavior when it receives different `type` props ("success"/"error"/"info") is that it produces a DOM element with different class names. It doesn't tell you anything about whether the component looks right on the screen, but it does confirm that the component is responding to the `type` prop correctly.