The element #player-seek-bar-progress used { left: 2.5% } so actually when the width of #player-seek-bar is 50rem, you will see a 2.5% margin so #player-seek-bar-progress is not actual 50rem, that's because we used a #player-seek-bar-indicator for indicator, and we want to put it in center of the start point when the duration is 0, so I guess we can just change 50rem to 52rem to fit the spec.