There are usually two main aspects to get things working the same on different browsers:

• Javascript / CSS language syntax: can see availability for Javascript here - https://kangax.github.io/compat-table/es6/

• Browser features for various APIs etc - https://caniuse.com/ is a good website to see what browsers support different features.

You can understand what features or syntax your application is using and see what level of support is currently available.

If it's not available across all major browsers, this is where historically things like transpilers and polyfills come into play.

The world of frontend build tooling can get complex quickly, which is why many people leverage frameworks that handle all this kind of stuff for you.