ckeditor5 strips classes from something like `<br class="mobile-break">`. This is problematic switching from ckeditor4. It can cause data loss on save when the WYSIWYG strips out critical classes that have ben applied.
Something that I think should work but doesn't seem to - If I add <br class> to the source editing box when configuring the text format.
I have uploaded a screen shot for the configuration that does not seem to work. I am wondering if the Manually editable HTML tags
is actually still getting filtered out by another plugin like "style" further down stream. IfI add the "style" plugin to the wysiwyg and add in the styles it still does not work (that would match our working ckeditor 4 implementation).
According to MDN: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/br - the <br>tag allows global attributes like `class`. The rich ckeditor4 and ckeditor5 demos also filter it out so the style box for the ckeditor4 implementation worked, where migrating those settings to ckeditor5 does not.