For all the people posting about Elsa making Anna's dress, the dress was already made and on a hanger waiting for Anna when Elsa woke her up. I'm sure the basic dress was woven and sewn in the usual way.



I don't have a link to it, but I saw something somewhere that showed a frame capture of what Elsa was doing with their dresses in Frozen Fever. She used a blast of icy wind to grab some flowers out of the vases and froze the flowers in place on their dresses using her ice as a sort of glue.



As for what sort of power Elsa has to transform her regularly woven and sewn coronation dress into her ice dress, I couldn't say. It doesn't make a whole lot of sense. It was a bit of artistic license and is one of those things it's best not to try to overthink. You see, the filmmakers wanted to transform Elsa's look to reflect the changes going on inside her, and they didn't want to interrupt her song or slow the action by having her dart behind a screen to change her dress and hairstyle in the ice palace far away from town when no one else was around, but they couldn't exactly show her stripping onscreen either. Besides, the transformation made for some gorgeous imagery.