What happens if you do not specify a transition duration?

If a transition duration is not specified, the transition will occur instantly. This means that when a property is changed in CSS, the effect will be applied immediately without any gradual transition effect. In scenarios where a transition is defined in CSS but lacks a duration, the outcome will be that the change happens right away, giving the impression that the transition is abrupt rather than smooth. It’s essential to define a duration to achieve a visually appealing transition effect over time.

In contrast, the other options imply that there would be either a default duration applied, the transition wouldn't take effect, or it would be ignored altogether, which is not how CSS transitions behave when the duration is absent. Therefore, specifying a duration is crucial for creating transitions that enhance user experience by allowing gradual visual changes instead of instant ones.