Hey @ all,
I think there is a fact about the designs that should be communicated within the documentation for creating or editing invoice-designs:
The header- and footer-sections have different CSS-IDs: header: #repeating-header, footer: #footer (!).
So if one like me wants to please one’s personal part-time-authism and change the ID #footer into something like a repeating-footer, it’s ruining the product-tables and text-formats within such (for example: no table-heads anymore or html-tags like aren’t recognized anymore).
It was a mess to find my mistake (but luckily I found it!).