Continuing the discussion from 
Introducing ESPCompose – A React/TypeScript-inspired UI toolkit for ESPHome:
I’ve added the starting pieces of a UI simulator into the ESPCompose toolkit. It was frustrating having to wait for a transpile + esphome compile + flash device before I could visually see my changes. 
With the espcompose simulate command, you can now run the transpile phase (fast) and hot reload a simulator UI that is hosted in a browser. The simulator supports running with a --ha-bridge mode that will connect to the a real HA instance. It’ll appear like an ESPHome node. If you run without the bridge, we create an internal mocked bridge to simulate entities.
It is very much still in the alpha stage, so expect bugs. I’d love to hear your feedback, bugs, and suggestions.
Enjoy!