@s_palm the alt text really should be a description of the image, it's not a place to put the punch-line of a joke :/ Here's a description that I came up with that could work here:

A cat is visible sitting inside the reflection of an empty cardboard box in a glass patio door.

@kepstin @s_palm

How about this for alt text?:

Photograph showing a box and it's reflection through a semi transparent mirror. A cat behind the mirror looks like it is in the reflection of the box, while the box itself appears empty.