The problem with .docx is that 'others' consider it standard and send you forms in that format to fill out. Preferably laid out using tables (within tables...) instead of setting tabs. I can tell you from experience that LO consistently screws up such layout.
My own word processing is in (Xe)LaTeX, using texworks and TeXlive students nowadays submit all their work in .docx as well; fortunately with minimal layout. Have never yet had occasion to prepare web content, so no HTML. Never looked into markdown either.