title: Preserve custom class name CKEditor5
body: <p>I using a plugin to add a custom class to the mediaEmbed. Works fine, but when I reload the page the editor filter the class name. The class name is applied to the <code>&lt;figure&gt;</code> html tag. I tried to apply follow the example here: <a href="https://ckeditor.com/docs/ckeditor5/latest/framework/guides/deep-dive/conversion/conversion-preserving-custom-content.html#loading-content-with-all-attributes" rel="nofollow noreferrer">https://ckeditor.com/docs/ckeditor5/latest/framework/guides/deep-dive/conversion/conversion-preserving-custom-content.html#loading-content-with-all-attributes</a> and while the class name is preserve the <code>&lt;figure&gt;</code> element is duplicated. Seem like there is not a simple solution of this.</p>