Steps:
1. Go to "Create" a new article
2. Click in the "intro" and type 'one'
3. Select 'Heading 1' from the toolbar
4. Press shift+enter and type 'two'
5. Press shift+enter and type 'three'
6. Click publish - there will be

onetwothree

If you do the same for "paragraph" preview will look fine:

one
two
three