The difference is that in the backend you have control over the runtime environment. In the frontend you don't. However, because frontend devs know that, they

The difference is that in the backend you have control over the runtime environment. In the frontend you don't. However, because frontend devs know that, they have build tools to support them. It started with jquery as an cross browser abstraction and has grown to a whole eco system ranging from websites like caniuse.com to editor plugins that check for features based on the current browser shares and your specific selection (e.g. browserlist) up to test-farm services like browserling. I think what makes frontend development harder, is that standards carry a lot of legacy (you can't just redesign CORS and 3rd party cookies) and that you have a very diverse set of runtime environments with very different capabilities.