MapCSS is the file format JOSM uses to express custom tag validation rules. mapcss-parse makes this format and the rules it can express available to JavaScript and node applications.
See here and here for more info about JOSM's use of MapCSS for tag validation rules.
the project was developed using node 10.7.0 as it makes use of JavaScript's awesome new Lookbehinds!
...on both operating systems
nvm install 10.7.0 && nvm use 10.7.0
yarn install # or npm install
yarn test # or npm test
yarn build # or npm run build