Well, for me it didn’t fully work on XWiki 11.10.3 either. Applying Paragraph Format (e.g. H3) first and then Style (e.g. success box) was loosing the heading format. Which is normal because the success box style is defined using a DIV.
When applying the Style first, e.g. success box, and then the Paragraph Format, e.g. H3, the previous output was:
<h3 class="box successmessage">test</h3>
while now you get:
Basically, in the previous versions the styles were preserved. But that doesn’t mean you were “able to have a title marked up as heading 1 (Paragraph format) inside an success box (Style).” because you didn’t have a heading inside a success box, you had a title styled as a success box.
To me this looks like a bug that was fixed in CKEditor. Basically if a success box is defined using a DIV, then replacing the DIV with a heading H3 doesn’t guarantee you that you still have the success box styles because the CSS may very well be written with specific selectors such as:
which don’t apply to headings. So I think CKEditor fixed this behavior because changing the format (e.g. go from DIV to heading or paragraph) doesn’t always preserve the style so they probably though it’s safer to remove the style when this happens.
In any case, this behavior is not controlled by XWiki. It comes from CKEditor. We can of course overwrite it but we try to avoid that so that we can upgrade more easily without breaking our code. The best option for you is to use the success box macro instead of the success box style.