I ran across a couple of issues:

* highlight to copy is broken (I assume because the source is not first-class text and they manipulate the clipboard with JS)

* failing to close the last list item in a ul causes the preview window to stop updating and highlights the next line in the editor but doesn't provide any indication of what is wrong

This is definitely not going to be sufficient for professional programming or web development but it looks and performs far better than similar projects I've seen in the past.