Something like this:
<!DOCTYPE html>
<html class="no-js" lang="en">
<head>
<title>Fullscreen Header Slider</title>
<meta charset="UTF-8">
<meta description="Fullscreen header">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<!-- Slider CSS -->
<link rel="stylesheet" href="http://wp.alveos.nl/demos/0.3.4/nagelstudio/wp-content/themes/wpa-nagelstudio/slider-assets/css/grid.css">
<link rel="stylesheet" href="slider-assets/css/slider.css">
<style>
html, body, div, span, object, iframe, h1, h2, h3, h4, h5, h6, p, blockquote, pre, abbr, address, cite, code, del, dfn, em, img, ins, kbd, q, samp, small, strong, sub, sup, var, b, i, dl, dt, dd, ol, ul, li, fieldset, form, label, legend, table, caption, tbody, tfoot, thead, tr, th, td, article, aside, canvas, details, figcaption, figure, footer, header, hgroup, menu, nav, section, summary, time, mark, audio, video {
margin:0;
padding:0;
border:0;
outline:0;
font-size:100%;
vertical-align:baseline;
background:transparent;
}
body {
line-height:1;
}
article, aside, details, figcaption, figure, footer, header, hgroup, menu, nav, section {
display:block;
}
nav ul {
list-style:none;
}
blockquote, q {
quotes:none;
}
blockquote:before, blockquote:after, q:before, q:after {
content:'';
content:none;
}
a {
margin:0;
padding:0;
font-size:100%;
vertical-align:baseline;
background:transparent;
}
/* change colours to suit your needs */
ins {
background-color:#ff9;
color:#000;
text-decoration:none;
}
/* change colours to suit your needs */
mark {
background-color:#ff9;
color:#000;
font-style:italic;
font-weight:bold;
}
del {
text-decoration: line-through;
}
abbr[title], dfn[title] {
border-bottom:1px dotted;
cursor:help;
}
table {
border-collapse:collapse;
border-spacing:0;
}
/* change border colour to suit your needs */
hr {
display:block;
height:1px;
border:0;
border-top:1px solid #cccccc;
margin:1em 0;
padding:0;
}
input, select {
vertical-align:middle;
}
/* Fullscreen Header */
#wpa-header {
width: 100%;
margin: 0 auto;
padding: 0;
height: 500px;
}
ul.slider {
margin:0;
padding:0;
list-style:none;
display:table;
width:100%;
border-collapse:collapse;
}
#wpa-header li {
width:16.66666%;/* 100% / 6 */
display:table-cell;
height: 500px;
overflow:hidden;
}
#wpa-header img {
object-fit: cover;
width:100%;
height:500px;
-webkit-filter: grayscale(100%);/*filter: grayscale(100%); breaks firefox */
}
#wpa-header ul:hover li {
width:10%;
}
#wpa-header ul li:hover {
width: 50%;
transition: width 1s ease;
cursor: pointer;
}
#wpa-header ul li:hover img {
-webkit-filter: grayscale(0%);
}
#wpa-header li:first-child img {
border: none;
}
#wpa-header .wpa-slide-image {
padding: 0 5px;
}
#wpa-header .wpa-border-color-light-gray {
border-left: 8px solid #CCC;
}
#wpa-header .wpa-border-color-dark-gray {
border-left: 8px solid #777;
}
#wpa-header .wpa-border-color-black {
border-left: 8px solid #000;
}
</style>
</head>
<body>
<section id="wpa-header">
<div class="col col-lg-8 col-md-8 col-sm-8 col-xs-8">
<ul class="slider">
<li><img src="http://wp.alveos.nl/demos/0.3.4/nagelstudio/wp-content/themes/wpa-nagelstudio/slider-assets/images/1-compressor.jpg" class="wpa-slide-image wpa-border-color-light-gray"></li>
<li><img src="http://wp.alveos.nl/demos/0.3.4/nagelstudio/wp-content/themes/wpa-nagelstudio/slider-assets/images/2-compressor.jpg" class="wpa-slide-image wpa-border-color-dark-gray"></li>
<li><img src="http://wp.alveos.nl/demos/0.3.4/nagelstudio/wp-content/themes/wpa-nagelstudio/slider-assets/images/3-compressor.jpg" class="wpa-slide-image wpa-border-color-black"></li>
<li><img src="http://wp.alveos.nl/demos/0.3.4/nagelstudio/wp-content/themes/wpa-nagelstudio/slider-assets/images/4-compressor.jpg" class="wpa-slide-image wpa-border-color-light-gray"></li>
<li><img src="http://wp.alveos.nl/demos/0.3.4/nagelstudio/wp-content/themes/wpa-nagelstudio/slider-assets/images/5-compressor.jpg" class="wpa-slide-image wpa-border-color-dark-gray"></li>
<li><img src="http://wp.alveos.nl/demos/0.3.4/nagelstudio/wp-content/themes/wpa-nagelstudio/slider-assets/images/6-compressor.jpg" class="wpa-slide-image wpa-border-color-black"></li>
</ul>
</div>
</section>
</body>
</html>
The filter kills firefox so I just left the webkit one in place.
You would still need some media queries just to reduce the height for smaller screens but the width is responsive.
(I think I misunderstood what you meant about slide and in any case that would leave a blank section if you simply slid them across without the others flexing also.)