user have many preferences, and sites/apps should try and conform to those as much as posible. ui color is a long standing one on the desktop. scrollbar behavior is another. personaly I like both up and down arrows on both the top and bottom of the bar as well as the abillity to click and drag and jumpto functionality. this is one element that draws me in or deters me from a particular ui

and when running across sites/apps that both change the look and the feel of my prefered settings (or at least the standard and expected behavior) I get annoyed.

lastly, scrollbars have historicly been a ui oddity from the developmet point of view. most ui’s didn’t treat them as standerd widgets or elivated them to some internal magic. this is ofcourse mostly due to to the lack of graphic acceleration needed to smoothly handle scrolling of large double buffered data. however most modern system have moved this off to the gpu.
which brings me back to my point (if I ever had one) that when sites/apps roll-their-own they tend to lack the default feel but the also tend to have serious performance issuse. which, to me, makes the entire product look bad.