From what I know it can't be "rotated losslessly" in all cases, only if the dimensions of the images are multiples of the MCU which are block of pixels whose size is determined by the chroma subsampling. Ex.: With the common "4:2:0" subsampling the MCU is 16x16 the image's height and width must be exactly multiples of 16, otherwise I think it's just visually lossless and uses some tricks that I'm still not sure how they work.