h1 are not wysiwyg

Reproduction steps

1. Edit a page and insert a h1 title
2. save and view

Expected

the h1 level is displayed with the same style as when editing

Actual

the h1 level is displayed in the same was as normal text

Analysis

I expect this to be on purpose by the default nextcloud style.
We need either to

1. adapt the editor style to show live to the user that h1 have a special style on NC
2. shift the heading level so that h2 are displayed for h1 and so on (and remove h6 possibly)