I'm out of merigue powder, is royal icing made with egg whites safe for holiday cookies?

Well people have been doing it that way for years... I think the risks are pretty small - if it's just for your family (not for sale) and there are no pregnant Moms, deathly ill seniors or little babies eating the cookies, you should be fine.

HOWEVER, make sure you don't used any cracked or otherwise damaged eggs to help reduce the risk of salmonella.

Also, you can buy pasteurized eggs (or just egg whites sold in cartons) which are also completely safe (just like meringue powder)