Linux: Abiword??
Abiword works fairly well for me on my more ancient Thinkpad. Opens and saves a range of file types but not markdown so does not meet criteria implied in the OA. Abiword can export LaTeX markup and so cross conversion to markdown possible using pandoc including proper quotation marks. Might even be lighter than an electron app. The CherryTree note taking program can edit rich text in a fashion and export to html and pdf.
Windows has Wordpad and Mac OS has TextEdit. Linux graphical layers/toolkits appear not to provide a rich text editor object so I suppose it is harder to provide a basic rich text native program under Linux. Yes, I do know that GNU Step might provide an early version of TextEdit. Not tried that yet.
PS: On Linux at home since 2006 or so. The only MacOS applications I miss are TextEdit and the amazing Preview (i.e. pdf as a display description language being used underneath).