In case any one was wondering the difference between "s++" and "++s" (the former of which is probably more common), prepending the "++" increments the variable before returning it, whereas appending it would increment after returning the value.
The code you posted is essentially equivalent to "s = s + 1; a.style.margin = s + e % 300".
The code you posted is essentially equivalent to "s = s + 1; a.style.margin = s + e % 300".