The basic idea behind Scrimba is that we record events instead of pixels. So what you're seeing when you're watching a Scrimba screencast is simply a re-creation of what the creator did when creating the screencast (which is done in-browser).
Even the interactions with the live preview are recorder, through a DOM-recorder we've built.
This opens up for tons of possibilities, including enabling the viewer to interact with the code.
We will add support for server side languages (e.g. Python, Ruby, Node) in the future. However, it's not at the top of our list, as there are several core things we need to fix. Once we have a timeline for that, it'll surely be shared on our Gitter channel:
Next up on our feature list is to make the creation process a lot easier, by enabling the creator to edit the recording after it's done. Will likely be launched next week :)
Happy to answer any questions people might have :)