The same written language?

[Seinfelddvd]

it is difficult to learn chinese

[mac123]

I have learnt that Taiwanese people often write Mandarin instead of chinese, although they still try to find ways to write down Taiwanese as it is spoken. But they are much more influenced by Mainland China than Hong Kong. So they learn Mandarin, the dialect of the Standard script. But the people in Hong Kong do not know or even learn Mandarin. So how do or can they write Cantonese in Mandarin style? Is it that they can't speak Mandarin but know only the grammar of it? Or is the grammar of Cantonese the same as Mandarin? And you told me that I could understand a Hong Kong based web page if I could speak Mandarin and knew the Trad. Char. So is that web page written in Mandarin style? Or is it written down Cantonese? And when are the Hong Kong supplementary characters are used ?
http://www.thewatchhut.co.uk/michael-kors.htm