Question: Our company develops ERP and CRM, and so far our products support IE and Firefox. Now we want to support Chrome, Safari and even Opera. Is there any comprehensive materials that introduce browser compatibility of JS and CSS? thks!

Answer: 
The purpose of import statements is just to make life easier for the human readers (and authors) of the code. Thus they are replaced by references to the fully qualified class/method names in the bytecode. And unused import statements are ignored.
`import` statements are only there for the compiler so it knows what class names (or static method names) you can access unqualified in your code (i.e. `MyClass` instead of `foo.bar.MyClass`). Behind the scenes this is just used to resolve to the fully-qualified class names which are then used in the bytecode as well.