No one enjoys hopping into a frigid vehicle on a cold morning, so a lot of people here in Illinois and throughout other cold-weather states choose to warm up their cars first. It makes for a nicer and more comfortable commute, but is doing that legal or illegal here in Illinois?
It's probably important to note that the question is "legal or not," rather than "is driving barefoot or in flip flops a good idea or a very bad idea in Illinois, or anywhere else?" The answer to one of those questions is a definite no.
The story of a little boy being left behind while his parents and siblings fly off to Paris is undoubtedly a heart-warming classic Christmas movie, but it probably wouldn't result in a "happily ever after" if it really happened here in Illinois.