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TinyMCE has various different settings to really customize its features. One of these very useful settings is the ability to provide your own custom CSS classes for the editor itself. That way if you have, for example, modified the style of heading1, then you can provide that CSS to tinyMCE; that way, when you are editing, you truly get the full power of the "WYSIWYG" as the heading1 will look the same in the editor as it will when the HTML is generated.