Reported on Meta.SE.

Is your feature request related to a problem? Please describe.

This markdown:

# one heading ## two heading ### three heading #### four heading ##### five heading ###### six heading 

renders this preview in the rich editor:

It gets posted like such:

Describe the solution you'd like

Fix the rendering in the editor such that h4-h6 are not shown as headings (or add h4-h6 support to SE, but that's out of scope here).