http://texts.io/ Came close.
I really hope that this project will develop into something better.
https://github.com/divbit/grimoire
currently supports image paste -> embedding (this was a crucial feature for me as well) and notes in markdown, latex, html, inline javascript (so you can do a note with some d3 or whatever).
I decided to not do live markdown preview because the save feature is fairly quick, and it gives more space on the page for editing on my small screen.
The split-pane approach, with one Markdown pane and one WYSIWYG pane, is such a waste of screen space, when self-previewing Markdown is all you need if you're not trying to create a published document.
A former Show HN project, bluedocs.io, offered this by default. But now it's defunct (and my notes are lost with it).
Of course, it's only an editor for local documents. I wonder if this could be glued to Standard Notes as a backend somehow.
Doesn't look like it supports tagging/searching and organising notes though (correct me if I'm wrong)
Sadly it's closed source and cost $19/user.