Browser APIs are less stable than you might think. If you've ever had to develop web apps for smart TVs (Samsung, LG, Vizio, etc.), then you'll find that each model year is frozen on an old version of Chromium – some from 2016, some from 2017, some from 2018, etc. There are enough differences between them to make this painful. For example: multiple versions of the Media Source Extensions API, web components, CSS variables, ES6 modules, and web font loading patterns.