style.css

@import url("https://fonts.googleapis.com/css2?family=Roboto:wght@400;700&display=swap");

* {
	box-sizing: border-box;
}

body {
	background: #11998e;  /* fallback for old browsers */
	background: -webkit-linear-gradient(to right, #38ef7d, #11998e);  /* Chrome 10-25, Safari 5.1-6 */
	background: linear-gradient(to right, #38ef7d, #11998e); /* W3C, IE 10+/ Edge, Firefox 16+, Chrome 26+, Opera 12+, Safari 7+ */
	display: flex;
	align-items: center;
	justify-content: center;
	flex-direction: column;
	height: 100vh;
	overflow: hidden;
	margin: 0;
}

.search {
	position: relative;
	height: 50px;
}

.search .input {
	background-color: #fff;
	border: 0;
	border-radius: 5px;
	font-size: 1.8em;
	padding: 15px;
	height: 75px;
	width: 75px;
	transition: width 0.3s ease;
}

.btn {
	background-color: #fff;
	border: 0;
	border-radius: 10px;
	cursor: crosshair;
	font-size: 2.4em;
	position: absolute;
	top: 0;
	left: 0;
	height: 75px;
	width: 75px;
	transition: transform 0.3s ease;
}

.btn:focus, .input:focus {
	outline: none;
}

.search.active .input {
	width: 400px;
}

.search.active .btn {
	transform: translateX(390px);
}

footer {
	position: absolute;
	bottom: 10px;
	font-size: 1.5em;
	font-family: 'Roboto', 'sans-serif';
	font-weight: 700;
	font-style: italic;
	opacity: .8;
	color: white;
}