Technically, I believe, you're still limited to 255 colors per frame- you can just put two frames with two different sets of 255 colors on top of each other with 0 delay. :)
However, due to historical reasons (GIFs optimized for old browsers on slow computers), modern browsers sometimes insert a small delay between image descriptors even if they're in the same frame. I generally consider this a bug. However, there are many broken GIFs out there...