Yes, the Tablet style is a lot better. Thanks!
I tried a longer text. There are still numerous problems. E.g., Aldiko does not respect my centered text style (style="text-align: center"), while MoonReader does. Both do not respect div-sections with margins (<div style="margin:20px;">), nor borders (style="border:solid gray thin"). On MoonReader, I have one page which cannot be viewed since it immediately flips to the next page. It has an image on it, which might be the reason.
I converted the text to the Kindle format using the Kindle Previewer. That seems to respect a lot more styles and looks way better. But, of course, even there are numerous hazards.
From the epubs I have seen, it seems to be difficult to publish technical texts or text with images. It is okay for novels. Using a PDF might be a solution. I have to consider this seriously.