HTML stops at h6, so 7-10 render as unknown elements editor.css makes blocks; their built-in styles continue level 6 at 12pt, since LibreOffice writes these styles with no properties and resolves them from its own pool. Both importers read a two-digit level again (the style-name regexes stopped at one digit). Verified through a LibreOffice re-save of both formats and in the live editor.