CSS transition from `display: none` on class change?

It does work when you remove the display properties.

#myelem {
    opacity: 0;
    transition: opacity 0.4s ease-in;
    -ms-transition: opacity 0.4s ease-in;
    -moz-transition: opacity 0.4s ease-in;
    -webkit-transition: opacity 0.4s ease-in;
}
#myelem.show {
    opacity: 1;
    transition: opacity 0.4s ease-out;
    -ms-transition: opacity 0.4s ease-out;
    -moz-transition: opacity 0.4s ease-out;
    -webkit-transition: opacity 0.4s ease-out;
}​

JSFiddle.

The reason for this is that only CSS properties with numbers can be transitioned. What do you think the “50% state” should be between “display: none;” and “display: block;“? Since that can’t be calculated, you can’t animate the display property.