Differences:

1. clientWidth is numeric, while getComputedStyle(elem).width returns a string with px at the end.
2. getComputedStyle may return non-numeric width like "auto" for an inline element.
3. clientWidth is the inner content area of the element plus paddings, while CSS width (with standard box-sizing) is the inner content area without paddings.
4. If there’s a scrollbar and the browser reserves the space for it, some browser substract that space from CSS width (cause it’s not available for content any more), and some do not. The clientWidth property is always the same: scrollbar size is substracted if reserved.