yarn install
yarn fetch-data
yarn start

Search for any path in the MDN JSON register e.g.

GET http://localhost:3000/javascript/classes/constructor
to get https://github.com/mdn/browser-compat-data/blob/master/javascript/classes.json#L57

or

GET http://localhost:3000/css/properties/animation-delay
to get https://github.com/mdn/browser-compat-data/blob/master/css/properties/animation-delay.json