I made a kind of todo/list pet project with html5 local storage earlier. All the front end was based on that storage, then it was only a matter of syncing. To do that I kept a queue in local storage, that I only emptied after a successful sync. Then you need to define conflict rules based on what you synced, and that is case-by-case.

For instance for me, deleting a task meant anything else regarding that task later was disregarded, it's deleted. Updates to tasks were treated by timestamp. For creating tasks, I had to have a temporary id that I updated after syncing, to a permanent id.

tldr; queue, precedence rules and timestamps.