New Post: Visual Editor additional tags

My problem is, that headlines are styled using an aditional wrapping div element.

Is this a requirement you cannot change? If you could change it so the markup instead is like this:

<h5 class="classname"> .... </h5>

In which case you can add support for this styling in common.xml. This config file will enable you to control class names and the creating on new elements, but not larger constructs like the div[@class='...']/h5 you describe.

(This feature request https://compositec1.codeplex.com/workitem/1237 should enable you to make this thing so consider upvoting that).

Also: As soon as I add a 'classes="headline"' to the <format> element of "h1" it is not visible anymore in the editor. Am I doing something terribly wrong?

you should format the element like this:

<format id="lead" label="Lead" image="" notes="" classes="lead" selector="p">
        <select />
    </format>

and place it within the <radiogroup /> element which has similar format elements (contains 'classes' attrib and not 'block' and 'inline' attribs).