Hong Kong written Chinese
Hong Kong written Chinese (HKWC) is a local variety of written Chinese used in formal written communication in Hong Kong. The common Hongkongese name for this form of Chinese is "written language" (), in contrast to the "spoken language" (), i.e. Cantonese. While, like other varieties of Standard Chinese, it is largely based on Mandarin it differs from the mainland’s national variety of Standard Chinese (Putonghua) in several aspects, for example that it is written in traditional characters, that its phonology is based on Cantonese, and that its lexicon has English and Cantonese influences. Thus it must not be confused with written Cantonese which, even in Hong Kong, enjoys much less prestige as a literary language than the "written language". The language situation in Hong Kong still reflects the pre-20th century situation of Chinese diglossia where the spoken and literary language differed and the latter was read aloud in the phonology of the respective regional variety instead of a national one. History 
With the establishment of Modern Standard Chinese in the Republic of China teaching materials began to be exported into the British Crown Colonies Hong Kong, Singapore and Malaya. The victory of the Communists over the nationalist Kuomintang in the Chinese Civil War, the retreat of the Republican government to the island of Taiwan, and the subsequent diplomatic isolation of the People's Republic of China under the One-China policy led to a diversification of the Standard Chinese language, so that there are four varieties of more strongly standardized Chinese today: Putonghua in Mainland China, Guoyu on Taiwan, and Huayu in Singapore and Malaysia. Hong Kong on the other hand was a British colony until 1997 and for most of the colonial era English had been the only official language. Chinese was only recognized as a co-official language in 1974 after recurring riots as well as scholarly activism. And although legal texts were translated from English into Chinese in the late 1970s the English versions alone continued to be the ultimately valid ones. This meant that there was not much historical effort on the British side to standardize Chinese in Hong Kong. This lack of political intervention facilitated the formation what was coined Hong Kong style Chinese (, translated as Hong Kong written Chinese by Shi, 2006) by linguist Shao Jingmin in 1996 in order to distinguish it from Putonghua. Development of registers 
Hong Kong written Chinese, if taken to mean all forms of Chinese writing employed in Hong Kong, has different registers depending on the context in which it is used. The high register used in government, schools, and formal settings, is the closest to Standard Chinese. Yet lower registers, used in more informal settings, also developed through an intermixture with written Cantonese. The rising popularity and prestige of the Cantonese language with the boom of the Hong Kong entertainment industry (especially cinema and music) in the 1980s was an important factor in the establishment of written Cantonese as a language used in subtitles, magazines, comic books, and popular fiction. In more formal settings, written Cantonese also appears in court protocols. After 1997 
Chinese, without specification of the variety, became a fully recognized official language of Hong Kong (besides English) with the handover to China in 1997. Since then Hong Kong written Chinese, or rather the high register, has been used on a local government level, while Putonghua is used in communications with the central government. In recent years the national government has promoted the use of Putonghua in Hong Kong as part of a process of political centralization while growing economic dependence on the mainland and migration into Hong Kong have increased the need for locals to learn the national standard. These and other political issues have led to tensions between mainlanders and locals trying to preserve the political and linguistic autonomy of Hong Kong. Phonology 
The grammar of the Hong Kong written Chinese is largely based on Mandarin Chinese, while its phonology is based on Cantonese. Yet one distinguishes between colloquial and literary readings of Chinese characters. The latter are based on traditional rime dictionaries and employed when reading out loud texts in Classical or modern written Chinese. Thus students in Hong Kong schools get corrected by their teachers when reading aloud HKWC texts in the colloquial pronunciation. Some regular differences between colloquial and literary readings in Cantonese are:

 The Late Middle Chinese (LMC) initial */fɦ-/ corresponds to colloquial /p-/ and /pʰ-/, but to literary /f-/, e.g. LMC */fɦuə̌/, coll. /pʰou˩˧/, and lit. /fu˩˧/, '(married) woman'. The LMC syllables /ŋiCnasal/ and /ŋi̯eCnasal/ correspond to colloquial /ŋɐCnasal/ and /ŋa:Cnasal/ respectively, but to literary /jɐCnasal/ and /jiCnasal/, e.g. LMC /ŋīm/, coll. /ŋɐm˨˩/, and lit. /jɐm˨˩/ 'to recite; to groan';  LMC /ŋi̯ên/, coll. /ŋaːn˨˩/, and lit. /jin˨˩/ 'research'. The colloquial rimes /-ɛːC/ and /-ɛːu̯/ correspond to literary rimes /-ɪC/ and /-iːu̯/ respectively, e.g. coll. /mɛːŋ˨˩/ versus lit. /mɪŋ˨˩/, 'name'. The colloquial rime /-ɐɪ̯/ corresponds to literary /-ɔɪ̯/ when going back to LMC */-ʌi/, whereas it corresponds to literary /-iː/ when going back to LMC */-i/. The colloquial rime /-œː/ corresponds to literary /-ɔː/. LMC syllables with voiced/breathy obstruent initials and rising-tone rimes correspond to colloquial readings with aspirate initials and low-rising-tone rimes, but to literary readings with tenuis initials and low-even-tone rimes, e.g. LMC /pɦə̌ŋ/, coll. /pʰaːŋ˩˧/, and lit. /paːŋ˨/. Here the colloquial variant preserves the phonetic realization of the LMC tone more authentically. The literary reading imitates the correspondence of LMC syllables with voiced obstruent initials and rising-tone rimes with Mandarin syllables with falling-tone rimes (the Cantonese mid- and lower-even tone rimes correspond to Mandarin falling-tone rimes). Lexicon

Lexical differences between HKWC, Putonghua, and Guoyu 
Some lexical differences between the varieties of Standard Chinese in the mainland (Putonghua), Hong Kong, and Taiwan (Guoyu):

Notes

Semantic differences between Putonghua and HKWC 
Some lexemes appear both in Hong Kong written Chinese and Putonghua, but may differ in their semantic range and value:

Notes

Comparison with written Cantonese 
Other than in its phonology, HKWC mostly differs almost as much from written Cantonese as Putonghua or Guoyu do. Some examples:

References 

Languages of Hong Kong
Mandarin Chinese
Chinese orthography
Cantonese culture