How to view an online markdown document in an online editor

StackEdit can do that but only with Markdown documents published on Gist. By the way, you can use StackEdit to publish your document on different websites.

Update:

StackEdit can open any URL within its viewer like that:
http://benweet.github.io/stackedit/viewer.html?url=http://path/to/file.md

Update:

I'm the developer of StackEdit.

Update:

To avoid browser caching of multiple pages, use: https://stackedit.io/viewer#!url=http://path/to/file.md

If you're only interested in viewing a markdown formatted document from a given URL have a look at Markdown Styler.

It supports appending the URL to your markdown document as GET parameter:

https://matteobrusa.github.io/md-styler/?url=https://my.site/my/url.md

Disclaimer: I'm the author of Markdown Styler, which I have created to show formatted markdown documents from Dropbox shared links.