When running on a desktop, you can easily resize the window
while the app is running to preview various screen sizes.
This, combined with hot reload, can greatly accelerate the
development of a responsive UI.

Solve touch first

Building a great touch UI can often be more difficult
than a traditional desktop UI due, in part,
to the lack of input accelerators like right-click,
scroll wheel, or keyboard shortcuts.

One way to approach this challenge is to focus initially
on a great touch-oriented UI. You can still do most of
your testing using the desktop target for its iteration speed.
But, remember to switch frequently to a mobile device to
verify that everything feels right.

After you have the touch interface polished, you can tweak
the visual density for mouse users, and then layer on all
the additional inputs. Approach these other inputs as
accelerator—alternatives that make a task faster.
The important thing to consider is what a user expects
when using a particular input device,
and work to reflect that in your app.