Shiba

Markdown editors, regardless of their complexity, can be easily divided into two main categories: with or without preview or live preview options.
If you prefer the simplicity and the distraction-free editing environments provided by Markdown editors without live preview, then you might consider giving Shiba a quick try.
Put simply, Shiba is a modern, Electron-based markdown preview utility that not only displays the changes of a specific document in realtime but it also points out possible errors, with basic linting capabilities.

Runs on all major OSs out there and does not require installation

The first thing you should know is the fact that since its built with Electron, Shiba works on all major OSs, namely Windows, macOS and Linux. Secondly, you will be happy to hear that the app does not require installation or any type of configuration on your part.
To get started with it, just click its executable file from the provided package and either make it watch a directory (by clicking the Folder-shaped button from the minimalist sidebar) or allow it to continuously monitor a specific file by dragging and dropping it on its main window.

Simplistic GUI and straightforward functionality

Evidently, after this, each and every modification performed to the Markdown file is automatically highlighted by Shiba within its simplistic interface. Considering that Shiba runs its linter engine automatically, each time it picks up errors or slight-mistakes it lets you know this fact by changing its button's color from yellow to red.
By clicking the aforementioned button, you can get access to all the details behind the errors via a retractable side panel. Its somewhat limited functionality also includes the possibility to copy the outline of window (basically it copies the Markdown content to your computer's clipboard) and to search for text.

Live markdown preview app that might seem obsolete for some while others might appreciate its simple functionality

Taking everything into consideration, it is safe to say that Shiba is a smooth-running application that provides a straightforward way to preview your Markdown documents and their changes in realtime, as well as find possible errors thanks to its built-in lint engine.
Before we end, we cannot but point out the fact that in spite of everything it offers, its usefulness is somewhat constrained since almost all modern Markdown editors bundle preview capabilities.
Nevertheless, users that prefer editing with the help of the Markdown language  while using simple text editors will surely appreciate Shiba's worth.