The content attribute is to contain the WebFinger handle of the author's fediverse profile, i.e. username and domain separated by an @-symbol. A leading @ can be included and ignored when processing. This format was chosen for its user-friendliness, as this style of handle is what users would be used to seeing and typing when using most fediverse software.

This choice was criticized a lot when the feature was introduced. Meta tags are primarily read by machines, not humans, so user-friendliness shouldn't be a concern. Reliability, on the other hand, is worth taking into consideration. preferredUsername and the user part of a handle can be changed at any time. Actor ID is a more stable identifier and in my opinion is a better match for the fediverse:creator tag.