Finding tags that are not defined in the schema

Is there an easy way/API to find tags that are in the rte but have NOT been defined in the schema?

What does ‘rte’ mean here?

Sorry about that! The rich text editor user interface (the content editable div) where the user is typing.

Unless something went wrong (as in a bug or a crash), the editor content will always conform to the schema.

We have the possibility that the user will enter in tags that won’t be a part of the schema and we don’t want to have them stripped them out – we want them to appear as text in the editor and be highlighted (basically to let the user know this needs to be fixed).