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To transition from one css value to another, you can use the transition attribute.

In your element, set the value you want to transition, with the time taken to do such: transition: flex-basis 0.3s; for example.

And the change flex-basis value somehow to see the transition, usually by changing either class or style property of an element.

You can change the type of transition by setting the transition-timing-function attribute: to ease-in-out for example.