One of the best thing about Emacs, that you can extend it's capabilities really well. I love writing in markdown and I use Emacs for that. Sometimes I want to see the text live that I am writing in a more readable way, than just markdown.
I recently discovered flymd. You can find it in MELPA.
M-x package-install RET flymd
After that you open your markdown file
And it will open it in your default browser window. I put the 2 windows side-by-side, so I can see how will it look like. The only problem is that I couldn't find how can I do CSS for the preview HTML file.
There are a couple extra things flymd has, like auto refresh and auto scroll, so if we have a long text, it will jump to a point where you are editing.