Basically all your numbers will be rounded to the nearest pixel.
You can't have a fraction of a pixel, but for all other measurements that are in the end converted to pixels, you can use decimal places.
However rounding errors can occur where by rounding up, you use more pixels than 100% (like 33%, 34%, and 33% together, at certain screen sizes, may fill one pixel more than 100%). This can cause problems in floated layouts. The easiest way to fix this is to leave a bit of extra space between elements (like use 33%, 33%, and 33% together, and leave the last 1%).