Q:

For Christmas Lights, is it safe to run extension cord under garage door?

For Christmas Lights over the garage, the nearest outlet is inside my garage. Is it safe to run an extension cord under the garage door to the outlet? Is there anything I should bear in mind while doing so (like specific types of outdoor cords)?
The garage type is one that has wheels on the side, attached to a metal track. It has a rubber seal at the bottom, which looks kinda like this. 

A:

Yes to the last: you certainly need outdoor-rated extension cords for anything exiting the house.
Now, as to how to run the cord:  the comments cover your planned implementation.  I would strongly recommend, however that you or your friendly electrician run wire (properly inside conduit) from the existing internal outlet to a new external outlet box.  The hardware cost is trivial, and the work involved in attaching to the existing wiring, drilling a small hole in the wall next to the garage doors (or whatever is the preferred location) is easy.  Plus you now have an outdoor outlet for general use.

