For example, I want to refer to someone on the internet, but I don't know this person's gender. Which personal-pronoun do I use? (as article I mean he, she, it, etc)
It's perfectly fine to use they in the singular sense. (Verbs are conjugated the same as they would be in the plural sense: "are they joking?", "did they break anything?".)
All the same, when the gender is not known, some authors prefer constructions like (s)he, he/she, he or she, she or he, etc., some prefer to stick with the masculine pronoun as default (he), others prefer to stick with the feminine pronoun for balance (she), some will alternate using he and she in their writing.
When talking about a hypothetical person, "they" is quite established usage, although "he or she" is more formal.
Someone posting a question should always clearly indicate the type of answer they want.
Someone posting a question should always clearly indicate the type of answer he or she wants.
However, you can risk offending someone if you use "they" about that person in particular -- almost as much as if you guessed and used the wrong gender. In that case, I would re-write the sentence to reuse the user name or to use other nouns:
As @ntrx clearly states in the question...
As the original poster clearly stated...