You can see the source code right next to the running page, and make edits that show up as you go. Super useful.
These websites let you load bundled libraries, like jQuery or React. However, many libraries expect users to install the library via npm, and don’t distribute bundled versions of themselves. That means they’re incompatible with these online editors.
Gomix is an online code editor from Fog Creek Software that came out today. It is totally rad and solves this problem.
For example, I created of a ProseMirror editor in the browser. You can see the code here. If you wanted to use this boilerplate, you can fork it (“remix” in Gomix’s vocab) to create a copy in your account and go wild.