I don't like "merge" word because it implies how the problem is solved. "Synchronise" would be better, as there is no merging, actually.
We use Operational Transformation, so all the changes are stored as operations (with their most important property being path - the position where the operation happened).
When a user comes online, they try to re-send all the operations that are buffered and not yet sent to the server. Then it is a matter of transforming these operations by the new operations that happened in the meantime (when the user was offline). Of course, the user also has to accept the new operations.
This is a bigger / more complex version of the basic problem of the real-time collaboration. During real-time collaboration with all users online you might need to transform your operation by maybe several other operations at most. When you go offline, it might be tens or hundreds of operations but the problem is basically the same, just bigger.
The quality of the transformation algorithms will make a huge role in how well the user intentions will be preserved.
"Platform is ready" means that the theory behind our solution is correct and that we checked some moderately complex scenarios.
Edit: still, the base scenario is that the users write "online" and "offline" kicks in when you lose a connection (for hours, even). If we are talking about "everyone writes offline and then they magically merge" then I think this might be a totally different feature (and maybe even outside of the editor and inside the CMS).
how did you solve the merge when somebody comes online again but creates a conflict with his changes?