IIRC slider progress is not yet a standardized CSS definition. Different browsers display it differently. Here is a CSS only solution for Chrome but it's a bit advanced.
codepen.io/ShadowShahriar/pen/zYPPYrQ
If you're not particularly comfortable with CSS, I highly recommend building your own slider bar with sprites and events. This way you can have much more control of the way it looks and behaves across different browsers as well. It will consist of three sprite objects - the left filled progress part, the handle, and the right unfilled part. You'll also want a few variables for minimum value, current value, and maximum value. Then you can use those values to adjust the size and position of each part.