A separator (HR) inserted at the beginning of a document is badly displayed and difficult to remove

That's a problem we'll potentially have with all block level elements.

I think we should insert a br or p before the HR when it's the first item inserted in the page from the WYSIWYG.
I have the problem with the table plugin too, a table inserted first means that you won't be able to put content above it.
AFAICS Google docs solve the issue by putting block level elements into divs so that you can still manage to put the caret between the div and the element.