trello.com, a.k.a. the "desktop" version, is device-agnostic so that it will actually work on any just about any laptop, smartphone, or tablet. You're getting the same code on every device, so it's cross platform in that sense. The custom scrollbars on the web are just a webkit thing. See: http://css-tricks.com/custom-scrollbars-in-webkit/
That said, we want to create the best experience for your device, and nothing beats the experience of a native app. Try them side-by-side. Each native app is written in its native language (Obj-C, Java, etc.). It's more code, but they just turn out better that way.