I think there a lots of ways to achieve it and it depends on your expectations.
We integrated HTML galleys in one of our journals, see e.g. View of In the Name of Civilisation. Sonja Schillings's <em>Enemies of All Humankind. Fictions of Legitimate Violence</em> | KULT_online For the editors one of the most important things was that the overall journal layout should be preserved when reading the HTML-fulltext.
By looking at the site source code you will see that we didn’t find a perfect solution but it works. Our approach is:
- create a very simple html file starting and ending with a html tag.
- Upload this file as galley then OJS will recognize it as html.
- we modified the htmlgalley plugin a little bit (see Problem styling HTML Galley OJS 3.0 at the end) so that header and footer stay.
But this is fine for short texts with just a few images. And we have a second html element in the page source which is forbidden. But browsers a nice enough to ignore it.