Munit runs fine with Haxe 4
Most unit testing frameworks don’t really care what you test or how your tests come up with results, so using an existing unittest framework will give you testrunners and reporting for free (some even have IDE integration and/or give you CI compatible reports)
Correct, usually targeting Chrome browser, but Firefox or Edge should also work.
selenium-standalone-x.x.x.jar is what you download from Selenium’s homepage, it’s what you would normally use if you wrote Selenium tests in a plain Java project.
By using -java-lib <lib.jar> Haxe will try to read that jar file during compilation and technically all types inside should automatically be available to your Haxe code, which means you don’t have to write externs.
Externs are comparably to typescript’s .d.ts files, where you tell Haxe that there is a type (e.g. a class or interface) with certain methods or fields that will be available at runtime. Since your externs contain names, parameters, types and return types Haxe can fully type your code, and trusts you that an external library containing all those types will be present when you run your project.
When writing externs you can concentrate on things that you actually use in your project so you don’t have to write full externs, you just need the parts that you use in your projects.
I think Java, C# and Flash are the only Haxe targets that support such automatic externs.