In my opinion, the coolest woman at the holiday soiree is rarely the one wearing the most obvious, on-the-nose party gown. She's the one who arrives in a perfectly slouchy satin blouse, an embellished skirt, or wearing a full suit with a tassel-adorned pouch bag—very much herself. It's a particular brand of effortless glamour that feels more fashion insider than festive dress code, and H&M's new holiday collection has it down to a science. For w…