如何在弹性盒中褪色?

How do I get a flex box to not be part of the page until I fade it in? I used to do this with 'display: 0;' and then use jquery .fadeIn(). But now if I set display to 0, when I fade it in, of course I lose the flex-iness of the box. If I use jquery to set display to flex, then it will just appear, not fade in. How do I get a flex box to not be part of the p