HTML, BODY, DIV, IMG {
	margin: 0;
	padding: 0;
	border: 0;
	}

HTML, BODY {
	width: 100%;
	height: 100%;
	}

* {
	box-sizing: border-box;
	}

/* Lives and Score at top left and right */
.hide_nav .stats_box { top: 0; }
.stats_box {
	position: fixed;
	top: 32px;
	z-index: 1;
	padding: 5px;
	font-family: sans-serif;
	font-size: 48px;
	font-weight: bold;
	color: red;
	}
.stats_box DIV:first-child {
	font-size: 50%;
	}
#lives_box {
	left: 0;
	text-align: left;
	}
#score_box {
	right: 0;
	text-align: right;
	}

/* Decorative background */
#background {
	width: 100%;
	height: 100%;
	}
#sky {
	position: relative;
	height: 60vh;
	background-color: skyblue;
	}
#land {
	position: relative;
	height: 40vh;
	background-color: green;
	}
#sun {
	position: absolute;
	width: 50px;
	height: 50px;
	right: 30%;
	top: 100px;
	}
.cloud {
	position: absolute;
	}
#cloud1 {
	top: -100px;
	left: 30%;
	width: 150px;
	height: 150px;
	}
#cloud2 {
	top: -25px;
	right: 20%;
	width: 100px;
	height: 100px;
	}
#cloud3 {
	bottom: 30px;
	left: -5%;
	width: 200px;
	height: 200px;
	}
#cloud4 {
	bottom: 30px;
	right: 9%;
	width: 75px;
	height: 75px;
	}
.tree {
	position: absolute;
	}
#tree1 {
	top: -110px;
	left: 15%;
	width: 150px;
	height: 150px;
	}
#tree2 {
	top: -40px;
	right: 10%;
	width: 50px;
	height: 50px;
	}
#tree3 {
	top: -45px;
	left: 60%;
	width: 60px;
	height: 60px;
	}

/* Airplane that flies across the sky */
#airplane {
	position: fixed;
	left: 120%;
	z-index: 2;
	width: 555px; 
	}
#airplane.airplane_fly {
	left: -555px;		/* hidden beyond left-hand edge */
	transition: left 10s linear;
	}
#airplane IMG {
	width: 200px;
	height: 100px;
	}
#airplane svg {
	display: inline-block;
	border: solid black 1px;
	width: 300px;
	height: 70px;
	margin-bottom: 20px;
	background-color: white;
	}
#airplane svg text {
	text-anchor: middle;
	font-size: 60px;
	font-family: sans-serif;
	}

/* Pictures on which the user can click to answer the question */
#picture_box {
	display: flex;
	flex-flow: row;
	justify-content: center;
	position: fixed;
	bottom: 0px;
	width: 100%;
	padding: 5vmin;
	}
.picture {
	border: 1px solid gray;
	border-radius: 10px;
	box-shadow: 1px 1px 4px 0 rgba(0,0,0,.75);
	width: 30vmin;
	height: 30vmin;
	margin: 0 4vmin;
	padding: 5px;
	background-color: white;
	}
.picture:hover {
	background-color: #c0c0c0;
	}
.picture:active {
	background-color: #808080;
	}

/* Popup with high score and Start button */
#start {
	position: fixed;
	top: 30%;
	top: calc(50vh - 100px);
	left: calc(50% - 150px);
	z-index: 10;
	background-color: #e0e0e0;
	color: red;
	border: solid 2px black;
	border-radius: 10px;
	box-shadow: 0 0 0 15px #e0e0e0;
	font-family: sans-serif;
	font-weight: bold;
	font-size: 30px;
	text-align: center;
	}
#start DIV {
	margin: 10px;
	}
#start_button {
	width: 300px;
	background-color: red;
	color: white;
	padding: 10px;
	border-radius: 10px;
	font-size: 60px;
	cursor: default;
	box-shadow: 1px 1px 4px 0 rgba(0,0,0,.75);
	}