PD4ML v4.0.6 introduces flexbox layout support.
Children of a flex container can “flex” their sizes, either growing to fill unused space or shrinking to avoid overflowing the parent. Both horizontal and vertical alignment of the children is supported.


Despite the message, the real validation error reason is a missing of <thead> section in a table. Add missing <thead> section to suppress the message. PD4ML creates an implicit <thead> section if leading table rows contain <th> cells only.
The message is clear: in PDF/UA you are not allowed to nest, let’s say, <h4> heading to <h2>, which happens quite often in real life HTML documents.


