How can I increase a scrollbar’s width using CSS?

This can be done in WebKit-based browsers (such as Chrome and Safari) with only CSS:

::-webkit-scrollbar {
    width: 2em;
    height: 2em
}
::-webkit-scrollbar-button {
    background: #ccc
}
::-webkit-scrollbar-track-piece {
    background: #888
}
::-webkit-scrollbar-thumb {
    background: #eee
}​

JSFiddle Demo

References:

• Custom Scrollbars in WebKit | CSS-Tricks
• WebKit scrollbar demo from CSS-Tricks
• 15 Different scrollbar configurations