How to animate the appearance of a modal window through the portal?

Question

css

Good afternoon!
In a react project, I'm making a modal window using portal.

You style animate keyframes to opacity from 0 to 1 in 0.5s.

But the modal window still appears instantly.
Is it possible to animate his appearance?

I know about ready-made libraries of modal windows, but I want to understand how to do this without using them.