* { box-sizing: border-box; } #rusty-microphone { max-width: 320px; } #pitch-indicator-bar-container { width: calc(100% - 2px); height: 30px; position: relative; background: rgb(200, 200, 200); } #pitch-indicator-bar { height: 100%; border: 1px solid black; width: 0; position: absolute; left: 50%; } #pitch-indicator-colours { width: 100%; height: 50px; } #flat-indicator { width: 50%; height: 100%; background: blue; float: left; } #sharp-indicator { width: 50%; height: 100%; background: red; float: left; }