@stumped
I didn't say, that it is easy to automate. But I'm sure, that it is possible to find a good solution for the task.
To your criticism:
bold, italic, italicbold
Indeed such visuell information should be kept.
Let's assum we have the following code:
<p>...<span class="foo">...</span>...</p>
.foo { font-family: "Arial Black" }
I suggest to keep a long list of bold fonts in the script. So that font would be recognized as bold.
The code will be replaced by:
<p>...<span class="foo">...</span>...</p>
.foo { font-weight: bold }
Sure, that is not perfect. And sure, that will not work in every situation. But in many. And that's what about I'm talking about. A good Plugin, with a focus on a narrow task. Not to much option. Clear.
Coming to your point justification:
I talk about eliminating the interference.
"left" is the standard. When you kill { text-align: justify } in the CSS you get back the option that the Kobo option "left" will have an effect. And in many cases, I think, the Kobo option "justified" willl work as well.
Do you know books, where it does work?
Coming to margins:
That's may be the most difficult part, that's true.
I hope for good heuristics, which work in many cases. And if not, the reader has to relinquish.
I understand your wish to clean an epub not only for a Kobo. But the task of cleaning a book is so complex, that I think it worth to focus on the usecases which are the most frequently. We will not be able to create a tool with a clear and simple UI, which could deal with all reader software of all books.
Well, it's nothing more, than my view.