Another interesting python3 based markdown editor is "retext" which is also in the Buster repositories.
It has webkit as the preview renderer and uses about 300Mb of ram.
The nice thing about it compared to ghostwriter is that it can show straight HTML in a preview pane. 
It has the same quirck all the others have: It cannot show or breaks at a direct external link.
- Until now only typora seem to be able to do that.
The winner up to now (in my book):
is still ghostwriter that only uses 100-200Mb ram but does bring in some Qt dependencies (especially the Qt web renderer), is noob friendly and has more options in what it can handle.
NB, @Vincent_Torri with ghostwriter available, I doubt an EFL based markdown editor will be any lighter.
Typora isn't too heavy with it's 200Mb Ram and very cool that it allows editing in the preview (and doesn't break on direct links) but alas not FOSS, so not a valid candidate for E16. 
