Emacs is widely available: Windows, mac.
Emacs adapts to your text, showing structure by coloring.
Here are a few settings that will improve emacs; once made, they look line this in the .emacs settings file
(setq auto-mode-alist (cons '("\\.md$" . html-mode) auto-mode-alist) ) (custom-set-variables '(column-number-mode t) '(indicate-empty-lines t) '(show-paren-mode t) '(show-trailing-whitespace t) '(size-indication-mode t) '(visible-bell t) )
show-trailing-whitespace switch is helpful
when writing markdown,
and markdown may have chunks of HTML embedded.