Rule of thumb: If it looks fine without any CSS, it's fine. Of course, rich content will never work without styling, but strive to make it look as good as possible without CSS.
The main difference between levels in font size. You should be fine, whatever you do with them, as long as you don't do
<h6>Page heading</h6>
<h5>Section heading</h5>
<h4>Subsection heading</h4>
Or weird stuff like that. h1
is supposed to be the biggest and most important, and while h6
(or something else, e.g. <bigheading></bigheading>
(yes, CSS will work on any tag)) might work if you style them right, they are not recommended or semantically correct.