style.css

html { box-sizing : border-box; }
html * { box-sizing: inherit; }

body {
	padding: 0;
	margin: 0;
	font-family: 'kanit', Arial, Helvetica, sans-serif;
	background-color: #ffffcc;
	text-align: center;
	display: flex;
	flex-flow: row wrap;
	justify-content: center;
	align-items: center;
	align-content: center;
	height: 100vh;
}

a {
	color:inherit;
	text-decoration: none;
}

.hidden {
	display: none;
}

.break {
	flex: 0 0 100%;
}

/* margins */
#startup, #title, #display, .button-group, img {
	margin: 5vmin;
}

#startup {
	flex-basis: 0 0 100%;
	cursor: pointer;
	font-size: 20vw;
	line-height: 20vw;
	font-weight: 900;
}

#startup:hover,
button:hover,
a:hover {
	z-index: 100;
	color: #ff9090;
	border-color: #ff9090;
	outline-color: #ff9090;
}

#title {
	font-size: 10vmin;
}

img {
	height: 10vmin;
	width: auto;
}

#player-box {
	font-size: 3vmin;
	width: 75vmin;
}

#controls {
	display: flex;
	justify-content: center;
}

.button-group {
	display: flex;
	justify-content: center;
}

#volume button {
	font-size: 5vmin;
	font-weight: 500;
}

#play-pause button {
	font-size: 3vmin;
	font-weight: bold;
}

#play-pause button[disabled], button:active {
	background-color: white;
}

[disabled] {
	color: black;
}

button {
	display: block;
	background-color: transparent;
	border:none;
	border: 0.2vmin solid black;
	outline: 0.2vmin solid black;
	font-weight: 500;
	height: 10vmin;
	width: 10vmin;
	user-select:none;
	position:relative;
	padding: 0;
	margin: 0;
	font-family: inherit;
}

#volumeDiv #volumeState {
	background-color: black;
	position: absolute;
	width: 100%;
	bottom: 0;
	left: 0;
}

#volumeDiv:hover #volumeState {
	background-color: #ff9090;
}