"Written English" comes in many flavors: mainly formal and informal.
Capitalization rules are strictly adhered to in formal English writing. In informal written text, literary works can exercise certain license (see a related recent ELU post, Milnesian Capitals). Communications and conversational English is not that strictly governed by rules, but follows conventions instead. The Internet has its own etiquette & style-conventions.
A word in all uppercase is not used in formal written English.
It is a 'style' that is essentially seen in real-time electronic conversation ('chat') as a desperate measure to make oneself "heard" above the "din" (chat messages of others in the "room"). This practice is strongly discouraged and considered 'rude'.