I have successfully implemented the demo widget and am now trying to offer different variants (classes) of the simple box, which can then be selected via styles, for example.

The result should then look like this:

<section class="simple-box variant-class">
    <h1 class="simple-box-title">Box title</h1>
    <div class="simple-box-description">
        <p>The description goes here.</p>
    </div>
</section>

I couldn’t manage it with the usual “style definitions”. In CKEditor 4 I solved this with CKEDITOR.dialogue.add[...] and made the class available for selection there, but that no longer exists.