<header id="header">
	<nav class="links" style="--items: 5;">
		<a href="#">Link 1</a>
		<a href="#">Link 2</a>
		<a href="#">Link 3</a>
		<a href="#">Link 4</a>
		<a href="#">Link 5</a>
		<span class="line"></span>
	</nav>
</header>

<div class="content">
	<strong>Pros</strong>
	<ul>
		<li>It's pure CSS.</li>
	</ul>
	<strong>Cons</strong>
	<ul>
		<li>It's pure CSS.</li>
		<li>All links need to have a consistent dimensions for this to work</li>
		<li>You have to set everything up yourself in terms of telling the nav how many items there are and where the <code>.line</code> should move to whenever one of the links gets hovered.</li>

	</ul>
	<strong>Notes</strong>
	<ul>
		<li>This uses Custom Properties, Calc, and Grid.</li>
		<li>While I couldn't find an easier way to make the line work with unevenly sized links, I tried my best to compensate for it by including the <code>--width</code> and <code>--left</code> custom properties, both of which would take priority over the current setup should either be defined.</li>
	</ul>
</div>
@import url('https://fonts.googleapis.com/css?family=Inconsolata|Lato:300,400,700');
html, body, h1, h2, h3, h4, h5, h6, p, li, ol, ul, pre {
	margin: 0;
	padding: 0;
}
html, body { min-height: 100%; }

body {
	background-color: #345;
	background-image: linear-gradient(to bottom, #0009, transparent);
	color: #eee;
	font-family: 'Lato', sans-serif;
	font-size: 62.5%;
	padding: 0 20px;
	box-sizing: border-box;
}
code {
	background: #fff1;
	font-family: 'Inconsolata', monospace;
	padding: .2em .4em;
}

.content {
	background-color: #fff;
	border-radius: 8px;
	box-sizing: border-box;
	color: #666;
	font-size: 1.6em;
	line-height: 1.4em;
	margin: 20px auto;
	margin-top: 80px;
	padding: 20px;
	width: 100%;
	max-width: 800px;
}
.content ul {
	margin: .5em 2em;
	padding: 0;
}

#footer {
	background-color: #246c;
	background-image: linear-gradient(to bottom, transparent, #0009);
	border-top: 1px solid #fff3;
	box-shadow: inset 0 1px 0 #fff3, 0 0 32px #000;
	overflow: hidden;
	padding: 8px;
	position: fixed;
	left: 0;
	right: 0;
	bottom: 0;
	z-index: 9001;
}
#footer a {
	color: #85c6f6;
	padding: 1em 0;
	text-decoration: none;
}
#footer ul {
	display: flex;
	list-style: none;
	justify-content: center;
	font-size: 2em;
	font-weight: 300;
}
#footer ul li {
	padding: 0 .5em;
}

/* Appearance */
.links {
	background-color: #123;
	background-image: linear-gradient(to bottom, #0003, transparent);
	border-bottom: 1px solid #0003;
	box-shadow: 0 0 32px #0003;
	font-size: 2em;
	font-weight: 300;
}
.links > a {
	color: #9ab;
	padding: .75em;
	text-align: center;
	text-decoration: none;
	transition: all .5s;
}
.links > a:hover {
	background: #ffffff06;
	color: #adf;
}
.links > .line {
	background: #68a;
	height: 1px;
	pointer-events: none;
}

/* The Magic */
#header {
	position: fixed;
	top: 0;
	left: 0;
	right: 0;
}
.links {
	display: grid;
	grid-template-columns: repeat(var(--items), 1fr);
	position: relative;
}
.links > .line {
	opacity: 0;
	transition: all .5s;
	position: absolute;
	bottom: 0;
	left: var(--left, calc(100% / var(--items) * (var(--index) - 1)));
	width: var(--width, calc(100% / var(--items)));
	--index: 0;
}
.links > a:hover ~ .line {
	opacity: 1;
}

.links > a:nth-of-type(1):hover ~ .line { --index: 1; }
.links > a:nth-of-type(2):hover ~ .line { --index: 2; }
.links > a:nth-of-type(3):hover ~ .line { --index: 3; }
.links > a:nth-of-type(4):hover ~ .line { --index: 4; }
.links > a:nth-of-type(5):hover ~ .line { --index: 5; }
.links > a:nth-of-type(6):hover ~ .line { --index: 6; }
.links > a:nth-of-type(7):hover ~ .line { --index: 7; }
.links > a:nth-of-type(8):hover ~ .line { --index: 8; }
.links > a:nth-of-type(9):hover ~ .line { --index: 9; }
.links > a:nth-of-type(10):hover ~ .line { --index: 10; }
.links > a:last-of-type:hover ~ .line { --index: var(--items); }
Run Pen

External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css

External JavaScript

  1. https://rockstarwind.github.io/codepen-footer/main.js