for instance if i write 我们明天可以去香港 (women mingtian keyi qu xianggang) in mandarin, if a cantonese speaking person sees this sentence, will he be able to understand it perfectly just as though it was cantonese (whichever way you pronounce it in cantonese)
in other words, is the primary major difference between mandarin and cantonese is its pronounciation. Is there also major differences in grammar and vocabulary because when i see written cantonese, the structure and vocabulary seems to be pretty much the same with mandarin