is there an equivalent of JSFiddle or Markdown for music notation? That would be useful for posting fragments of notation into StackExchange questions, as well as other blogs/wikis/webpages. math.stackexchange.com does something similar with MathJax for equations.
SE Music Practice and Theory supports ABCjs.
X:1 T:Spiffy Example K:none M:4/4 L:1/4 c/4 | c4 |] w:ta | da! |]
Additionally, you can use an external editor, with a newer version of the library, to write things out.
My VexFlow is the closest thing I can think of to being "JSFiddle for music notation". You need to create an account before using the site, but it's free and lets you create notation snippets that are then hosted on the site and can be viewed by anyone you give the link to.
In general, it's more geared toward writing articles with notation examples embedded (it supports Markdown syntax for text), but it works just fine for standalone music snippets too. It uses VexTab as the format for entering music, which can display both Western notation and guitar tablature.
I've put together a simple example named What's the name of this tune? to demonstrate what the end result might look like.