If you have, dear reader, bookmark the crap out of this page. These are up-to-date (2019) guidelines and best practices for font sizes across all major platforms – iOS 12, Android/Material Design, and responsive web. Here’s a handy table of contents for ya:

A mostly reasonable approach to JavaScript

Note: this guide assumes you are using Babel, and requires that you use babel-preset-airbnb or the equivalent. It also assumes you are installing shims/polyfills in your app, with airbnb-browser-shims or the equivalent.