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An egregious understatement of what Composer does is that it downloads files. If you need to reference some of those files in your application, notably Javascript or CSS or other client-side assets, then yes: you still have to code that into your application. The bit that Composer does automatically is for PHP classes, which is to create a single file you can include (again, you have to make your application do it) that will automatically pull in whatever classes the libraries provide. To learn how to use x3dom, I suggest starting with the tutorial.