Is one allowed to use capitalization for emphasis?

In non-electronic written English, capitalization is rarely used for emphasis. Much more common is italics or underlining. While underlining was very common in the age of the typewriter, word processing has made italics more accessible (it has long been the preferred technique for emphasis in printed materials).

In email and other electronic communication, capitalization is sometimes used, but it is generally considered akin to a loud voice, rather than an inflected voice. An entire sentence in caps is often called shouting, and is strongly discouraged.

It seems to me that the use of caps to indicate "shouting" is a matter of subjective preference. While there are cases when the overuse of caps is glaring, other times, it would seem to be more a matter of opinion. I try to refrain from the use, except for an occasional word or phrase. They are rendered more effective, in my OPINION, when they are employed with restraint.

"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'.