Most design tools only approximate how the end result will look and feel. Will typography render as intended? Is that animation smooth or kinda janky? Does that toolbar feel weird when the virtual keyboard is visible? Is this idea even feasible?
When I’m already working in HTML and CSS, there’s no guessing. I immediately experience the strengths and weaknesses of the medium firsthand, and I can adapt to that reality in the moment instead of having to compromise much later in the process.
While I’m not always thinking about the design aspect as much in my day-to-day work, the idea reminds me very much of how I prototype. Often I am tasked with spikes at work to figure out how to do something or if it can be done. All the documentation in the world doesn’t beat using the documentation to build and experiment with a small proof of concept to get your hands dirty and to see what can be done.