DWS compiler and execution engine are reaching very nice maturity levels, meaning they can be used to compile & execute code interactively, for instance to edit a web page, which mixes HTML, CSS, server-side dynamic Object Pascal content generation and client-side Object-Pascal scripted content.
The minimalistic Test app used in the video is less than 50 lines of code, and leverages DWScript [1], SynEdit [2], & ChromiumEmbedded [3].
And yes, you’re allowed to make mistakes, have infinite loops and other coding horrors. Not that you should ;-), but while entering code, they’ll most certainly happen.