There are a few potential feature and refactoring improvements to consider:

• We want to add a Lit directive for more ergonomically observing elements. This could be a separate module that provides an observeWith(observer) directive. This approach allows the controllers to be used without loading lit-html, but it's a little unergonomic v. something like ${controller.observe()}. If we do the latter, we'll need to disambiguate between using observe as a directive and imperatively. This could be done by checking if an argument is provided.
• Refine the types associated with the callback and value. These are currently just marked as unknown but they could be handled with a generic. Working this out with the tests will take a bit of refinement.
• The 4 observer controllers could share a base class. They are similar but each have some differences which need to be carefully considered. There's definitely an opportunity for more code sharing, however.