Greetings @florianruckelshausen,
The solution, which worked for me, was adding this variable into HTML plugin php file: Add head elements from HTML Galley file by Vitaliy-1 · Pull Request #1219 · pkp/ojs · GitHub
Than adding this line to display.tpl (in htmlArticleGalley): {$htmlGalleyContents}
So all your html article content will be downloaded.