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)

  '(column-number-mode t)
  '(indicate-empty-lines t)
  '(show-paren-mode t)
  '(show-trailing-whitespace t)
  '(size-indication-mode t)
  '(visible-bell t)

The show-trailing-whitespace switch is helpful when writing markdown, and markdown may have chunks of HTML embedded.