Standard HTML (when used to add different heading levels in the header of a table) does not render properly in the rich text view. Moreover, switching back to Markdown completely destroys the table:

It only renders the HTML after posting the answer (provided you've not switched back to Markdown before posting).

Test:

Compare with the old editor (the HTML is properly rendered in the preview):

Also reported in this answer to Opt-in alpha test for a new Stacks editor.