Comment by crabmusket

Browsers themselves aren't usually the problem. While sometimes they make changes, like what APIs are available without HTTPS, I think you're right about their solid backwards compatibility.

What people really mean when they talk about the frontend is the build system that gets your (modern, TypeScript) source code into (potentially Safari) browsers.

Chrome is highly backwards compatible. Webpack, not so much.

This build system churn goes hand-in-hand with framework churn (e.g. going from Vue 2 to 3, while the team have put heaps of effort into backwards compatibility, is not issue-free), and more recently, the rise of TypeScript and the way the CJS to ESM transition has been handled by tools (especially Node).