Using CKEditor 4.2.2
Setup rules for allowed content (with the global allowedContent property). Works great. Anybody created a user-friendly warning when HTML attributes, classes, styles and elements are automatically removed when not complying to the content rules? Otherwise HTML settings are removed without the user be made aware.

That sounds like it could be
That sounds like it could be a cool new feature request. You should post it on the dev site.
Customer and Community Manager, CKSource
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Forking an edit
Looks like in the dev filter.js there is a check funtion that looks at HTML elements, attributes, styles and classes returning a thumb up or down. Printing stuff to the console isn't "public" enough for normal users. What if the Editor itself hade a type of console or friendly "messages" that faded after x-number of seconds?
Need a day or two to settle into how allowedContent is internally used. But my genereal problem is really getting out messages. The need for a "message" bar or "message" handler in the editor.
What do people think?