Libel can occur online as well as in newspapers and other printed material.
Knowledge of the 'defamed' person is immaterial.
The reality is there are very few defamation cases whether it be slander or libel - it is too expensive and unless the 'defamer' has money it is totally pointless; monetary compensation is the only redress.
And of course any court action revives the original defamation, possibly aggravating the situation for the innocent person and his or her family.
Unless the damage caused by the defamation is great, it is rarely worth pursuing action.
Even if the claimant is successful s/he may not be awarded costs - there was a case where the compensation awarded was 1p, no costs. The legal bill was many thousands of pounds, which had to be paid by the claimant