The elements I set the Susy span-column()
mixin to, do not obey to it. They are misaligned ever so slightly. How do I achieve perfect alignment of my HTML eleme
Susy grids are (by default) fluid on the inside. All fluid grids suffer from some amount of sub-pixel rounding on the browser end. That said, what you are mainly seeing is sub-pixel rounding on the background grid, which is normally worse than the actual element-rounding. That is noted in the docs - the grid background is a rough guide, not a pixel-exact ruler.
Susy also has isolation options, which can be used to stop sub-pixel errors from compounding.