Inksplat Web Design

Thanks for your help I only tested it in firefox, turns out that's the problem

Hi I coded this animated boombox in notepad++ and when the dragger at the bottom is dragged the width of the slider should increase, like in the image below. It works perfectly in my browser but when I put the same code in codepen the width updates as its supposed to but the slider never animates. Is this a codepen issue? or is there something I've left out that is causing this? Thanks