Nice demo by Lim Chee Aun, showing how you can use 3D Touch on the web in a simple drawing app:

This is a demo for 3D Touch on Mobile Safari on iPhone 6S and 6S Plus. It’s a simple line drawing app which you can use the force of the finger to control the width of the drawn line.

Mobile Safari on iOS 9 for iPhone 6S and 6S Plus introduces 3D Touch for web developers. The line drawing code is heavily inspired by (or copied from) @kangax’s 2013 article: Exploring canvas drawing techniques.

Make sure to apply different levels of pressure to get thicker lines. You can play around with the demo here.